Control default repositories via meta-build (and the YAML-header)

[lefou]

It looks like, we currently can't control the default repositories via a YAML header. But this is, what most users want.

E.g. to use a Scala 3.8 nighly release, one has to use a dedicated repository, but this doesn't work:

//| repositories:
//| - https://repo.scala-lang.org/artifactory/maven-nightlies

Probably, because the repository is not only required in ScalaModule but also in the JvmWorker.