build: Run instrument plugin with toolchain

# Conflicts:
#	buildSrc/src/main/groovy/MuzzlePlugin.groovy

Author: smola

buildSrc/src/main/groovy/InstrumentPlugin.groovy

@@ -152,17 +152,16 @@ abstract class InstrumentTask extends DefaultTask {
   }
 
   private workQueue() {
-    if (this.javaVersion) {
-      def javaLauncher = this.javaToolchainService.launcherFor { spec ->
-        spec.languageVersion.set(JavaLanguageVersion.of(this.javaVersion))
-      }.get()
-      return this.workerExecutor.processIsolation { spec ->
-        spec.forkOptions { fork ->
-          fork.executable = javaLauncher.executablePath
-        }
+    if (!this.javaVersion) {
+      this.javaVersion = "8"
+    }
+    def javaLauncher = this.javaToolchainService.launcherFor { spec ->
+      spec.languageVersion.set(JavaLanguageVersion.of(this.javaVersion))
+    }.get()
+    return this.workerExecutor.processIsolation { spec ->
+      spec.forkOptions { fork ->
+        fork.executable = javaLauncher.executablePath
       }
-    } else {
-      return this.workerExecutor.noIsolation()
     }
   }
 }

buildSrc/src/main/kotlin/datadog/gradle/plugin/muzzle/MuzzleTask.kt

@@ -40,12 +40,17 @@ abstract class MuzzleTask : DefaultTask() {
       val javaLauncher = javaToolchainService.launcherFor {
         languageVersion.set(JavaLanguageVersion.of(muzzleDirective.javaVersion!!))
       }.get()
+      // Note process isolation leaks gradle dependencies to the child process
+      // and may need additional code on muzzle plugin to filter those out
+      // See https://github.com/gradle/gradle/issues/33987
       workerExecutor.processIsolation {
         forkOptions {
           executable(javaLauncher.executablePath)
         }
       }
     } else {
+      // noIsolation worker is OK for muzzle tasks as their checks will inspect classes outline
+      // and should not be impacted by the actual running JDK.
       workerExecutor.noIsolation()
     }
     workQueue.submit(MuzzleAction::class.java) {