Fix docjar context classloader conflict in scala 3

[lihaoyi]

Fixes #4316

This isn't a general solution to make context classloaders behave properly in the presence of long-lived worker objects instantiated from classloaders. Rather, we just hard-code support in the Scala3 docjar callsite to properly set the context classloader there, since these context-classloader-sensitive pieces of code are pretty rare in Mill and so we can just deal with them individually as they pop up

CC @joan38