Re-Introduce workaround to fetch scala source
Re-Introduce workaround to fetch scala source

This is currently failing in the CI (with Scala 2.12.7).

6360501b95cbaaa4e7e310311a72b2b752f6083a erroneously removed this
workaround: My local tests were incorrect.

I have manually checked that `sbt ++2.12.7 scalalib/fetchScalaSource`
works again after this commit.

gzm0/scala-js,sjrd/scala-js,sjrd/scala-js,scala-js/scala-js,gzm0/scala-js,scala-js/scala-js,sjrd/scala-js,scala-js/scala-js