Fix classloading failure when rebootstrapping (#5136)

This seems to fix the classloading problems we were having in
#5132; reproduced the problem by
doing
```
rm -rf out 
./mill dist.installLocal
rm -rf out 
./mill-assembly.jar '{main,scalalib,testrunner,bsp,testkit}.__.test'
```

And repeating that workflow on top of this PR seems to fix it. Must be
some funky interaction between the class classloader and the context
classloader that causes the lazy instantiation of
`mill.api.SystemStreams.ThreadLocalStreams` to fail during classloading,
but forcing the classloading to happen ahead of time sand just keeping
the long-lived reference seems to work. I haven't actually managed to
understand the root cause so the workaround will have to do for now

```
bsp.worker.docJar java.lang.reflect.InvocationTargetException
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    java.base/java.lang.reflect.Method.invoke(Method.java:569)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$2(ZincWorkerImpl.scala:262)
    scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.scala:17)
    mill.api.Retry.$anonfun$apply$1(Retry.scala:32)
    mill.api.Retry.$anonfun$apply$1$adapted(Retry.scala:32)
    mill.api.Retry.rec$1(Retry.scala:38)
    mill.api.Retry.rec$1(Retry.scala:50)
    mill.api.Retry.rec$1(Retry.scala:50)
    mill.api.Retry.indexed(Retry.scala:54)
    mill.api.Retry.apply(Retry.scala:32)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$1(ZincWorkerImpl.scala:232)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$1$adapted(ZincWorkerImpl.scala:228)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$withCompilers$2(ZincWorkerImpl.scala:479)
    mill.api.CachedFactory.withValue(CachedFactory.scala:39)
    mill.scalalib.worker.ZincWorkerImpl.withCompilers(ZincWorkerImpl.scala:478)
    mill.scalalib.worker.ZincWorkerImpl.docJar(ZincWorkerImpl.scala:228)
    mill.scalalib.ScalaModule.packageWithZinc$1(ScalaModule.scala:317)
    mill.scalalib.ScalaModule.$anonfun$docJar$2(ScalaModule.scala:410)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:245)
java.lang.NoClassDefFoundError: mill/api/SystemStreams$ThreadLocalStreams$
    mill.api.SystemStreams$ThreadLocalStreams$Err$$anon$2.delegate(SystemStreams.scala:151)
    mill.api.SystemStreams$ThreadLocalStreams$Err$$anon$2.delegate(SystemStreams.scala:151)
    mill.api.SystemStreams$ThreadLocalStreams$ProxyOutputStream.write(SystemStreams.scala:156)
    java.base/java.io.PrintStream.write(PrintStream.java:568)
    java.base/sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:234)
    java.base/sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:313)
    java.base/sun.nio.cs.StreamEncoder.implFlush(StreamEncoder.java:318)
    java.base/sun.nio.cs.StreamEncoder.flush(StreamEncoder.java:160)
    java.base/java.io.OutputStreamWriter.flush(OutputStreamWriter.java:248)
    java.base/java.io.BufferedWriter.flush(BufferedWriter.java:257)
    java.base/java.io.PrintWriter.newLine(PrintWriter.java:567)
    java.base/java.io.PrintWriter.println(PrintWriter.java:710)
    java.base/java.io.PrintWriter.println(PrintWriter.java:821)
    scala.tools.nsc.reporters.ConsoleReporter.finish(ConsoleReporter.scala:30)
    scala.tools.nsc.ScalaDocReporter.finish(ScalaDoc.scala:87)
    scala.tools.nsc.ScalaDoc.process(ScalaDoc.scala:57)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    java.base/java.lang.reflect.Method.invoke(Method.java:569)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$2(ZincWorkerImpl.scala:262)
    scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.scala:17)
    mill.api.Retry.$anonfun$apply$1(Retry.scala:32)
    mill.api.Retry.$anonfun$apply$1$adapted(Retry.scala:32)
    mill.api.Retry.rec$1(Retry.scala:38)
    mill.api.Retry.rec$1(Retry.scala:50)
    mill.api.Retry.rec$1(Retry.scala:50)
    mill.api.Retry.indexed(Retry.scala:54)
    mill.api.Retry.apply(Retry.scala:32)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$1(ZincWorkerImpl.scala:232)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$1$adapted(ZincWorkerImpl.scala:228)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$withCompilers$2(ZincWorkerImpl.scala:479)
    mill.api.CachedFactory.withValue(CachedFactory.scala:39)
    mill.scalalib.worker.ZincWorkerImpl.withCompilers(ZincWorkerImpl.scala:478)
    mill.scalalib.worker.ZincWorkerImpl.docJar(ZincWorkerImpl.scala:228)
    mill.scalalib.ScalaModule.packageWithZinc$1(ScalaModule.scala:317)
    mill.scalalib.ScalaModule.$anonfun$docJar$2(ScalaModule.scala:410)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:245)
java.lang.ClassNotFoundException: mill.api.SystemStreams$ThreadLocalStreams$
    mill.api.SystemStreams$ThreadLocalStreams$Err$$anon$2.delegate(SystemStreams.scala:151)
    mill.api.SystemStreams$ThreadLocalStreams$Err$$anon$2.delegate(SystemStreams.scala:151)
    mill.api.SystemStreams$ThreadLocalStreams$ProxyOutputStream.write(SystemStreams.scala:156)
    java.base/java.io.PrintStream.write(PrintStream.java:568)
    java.base/sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:234)
    java.base/sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:313)
    java.base/sun.nio.cs.StreamEncoder.implFlush(StreamEncoder.java:318)
    java.base/sun.nio.cs.StreamEncoder.flush(StreamEncoder.java:160)
    java.base/java.io.OutputStreamWriter.flush(OutputStreamWriter.java:248)
    java.base/java.io.BufferedWriter.flush(BufferedWriter.java:257)
    java.base/java.io.PrintWriter.newLine(PrintWriter.java:567)
    java.base/java.io.PrintWriter.println(PrintWriter.java:710)
    java.base/java.io.PrintWriter.println(PrintWriter.java:821)
    scala.tools.nsc.reporters.ConsoleReporter.finish(ConsoleReporter.scala:30)
    scala.tools.nsc.ScalaDocReporter.finish(ScalaDoc.scala:87)
    scala.tools.nsc.ScalaDoc.process(ScalaDoc.scala:57)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    java.base/java.lang.reflect.Method.invoke(Method.java:569)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$2(ZincWorkerImpl.scala:262)
    scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.scala:17)
    mill.api.Retry.$anonfun$apply$1(Retry.scala:32)
    mill.api.Retry.$anonfun$apply$1$adapted(Retry.scala:32)
    mill.api.Retry.rec$1(Retry.scala:38)
    mill.api.Retry.rec$1(Retry.scala:50)
    mill.api.Retry.rec$1(Retry.scala:50)
    mill.api.Retry.indexed(Retry.scala:54)
    mill.api.Retry.apply(Retry.scala:32)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$1(ZincWorkerImpl.scala:232)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$docJar$1$adapted(ZincWorkerImpl.scala:228)
    mill.scalalib.worker.ZincWorkerImpl.$anonfun$withCompilers$2(ZincWorkerImpl.scala:479)
    mill.api.CachedFactory.withValue(CachedFactory.scala:39)
    mill.scalalib.worker.ZincWorkerImpl.withCompilers(ZincWorkerImpl.scala:478)
    mill.scalalib.worker.ZincWorkerImpl.docJar(ZincWorkerImpl.scala:228)
    mill.scalalib.ScalaModule.packageWithZinc$1(ScalaModule.scala:317)
    mill.scalalib.ScalaModule.$anonfun$docJar$2(ScalaModule.scala:410)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:245)
```

Hopefully after merging this into `0.12.x` and cutting an dev release,
rebootstrapping should work again

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: lihaoyi

main/api/src/mill/api/SystemStreams.scala

@@ -147,8 +147,14 @@ object SystemStreams {
   private[mill] object ThreadLocalStreams {
     val current = new DynamicVariable(original)
 
-    object Out extends PrintStream(new ProxyOutputStream { def delegate() = current.value.out })
-    object Err extends PrintStream(new ProxyOutputStream { def delegate() = current.value.err })
+    object Out extends PrintStream(new ProxyOutputStream {
+          val current = ThreadLocalStreams.current //
+          def delegate() = current.value.out
+        })
+    object Err extends PrintStream(new ProxyOutputStream {
+          val current = ThreadLocalStreams.current
+          def delegate() = current.value.err
+        })
     object In extends ProxyInputStream { def delegate() = current.value.in }
 
     abstract class ProxyOutputStream extends OutputStream {