[backport] SI-7391 Use ForkJoin in Scala actors
... on Java 6 and above, but keep honoring the
actors.enableForkJoin system property.

Like SI-7236 and SI-7237, the logic in
scala.actors.scheduler.ThreadPoolConfig.useForkJoin
(which resulted in a different thread pool implementation
being chosen) was causing random hangs in the test
concurrent-stream.scala when running on Avian.