Fix #11076: Use bootstrapped scalaInstance for community build

[griggt]

This PR seems to fix the issue, however it is not at all clear to me whether there are better/alternative solutions.

Fixes #11076

[nicolasstucki]

I double-checked that the fix works for the case I encountered. It looks good. Not sure if there is a better alternative. @smarter WDYT?

[smarter]

The problem is that the community-build project uses a macro: https://github.com/lampepfl/dotty/blob/master/community-build/src/scala/dotty/communitybuild/FieldsImpl.scala, macros require classloading which means the version of the standard library in the scalaInstance must match the one used in the project (so bootstrapped in both case), which in turns means that all bootstrapped projects have to be compiled. This is fine for the community-build since we need the bootstrapped projects compiled anyway to run it, so we can make the change proposed in this PR but it would need to be documented better (and ideally it should avoid duplicating code and comment with the other scalaInstance). Alternatively, we could get rid of the macro in the community-build project: IMO it's really overkill to use a macro here, the list of project could be built by adding entries into a mutable set for example.

[nicolasstucki]

We should get rid of the macro and just list them. We already list those in other parts of the code.

[nicolasstucki]

I added a commit to remove the macro in #11074