Use en_US.UTF-8 locale in scalalib tests on macOS (#4747)

Seems C.UTF-8 isn't available there

Author: alexarchambault

scalalib/package.mill

@@ -1,4 +1,5 @@
 package build.scalalib
+import scala.util.Properties
 import scala.util.chaining._
 import coursier.maven.MavenRepository
 import mill._
@@ -29,7 +30,10 @@ object `package` extends RootModule with build.MillStableScalaModule {
   }
   def testIvyDeps = super.testIvyDeps() ++ Agg(build.Deps.TestDeps.scalaCheck)
   def testTransitiveDeps = super.testTransitiveDeps() ++ Seq(worker.testDep())
-  def testForkEnv = super.testForkEnv() ++ Map("LC_ALL" -> "C.utf8")
+  def testForkEnv = {
+    val locale = if (Properties.isMac) "en_US.UTF-8" else "C.utf8"
+    super.testForkEnv() ++ Map("LC_ALL" -> locale)
+  }
 
   object backgroundwrapper extends build.MillPublishJavaModule with build.MillJavaModule {
     def ivyDeps = Agg(build.Deps.sbtTestInterface)