Fix path to Hadrian build script for GHC source builds

[fwcd]

The build script has been renamed from build.stack.sh to build-stack (and build-stack.bat on Windows).

Fixes #5528.

• Any changes that could be relevant to users have been recorded in the ChangeLog.md (no user-facing changes, so not necessary)
• The documentation has been updated, if necessary.

The fix has been tested manually with recent GHC versions.

[snoyberg]

My only concern is that this seems like it will break old builds with old versions of GHC. I'm not sure if we really care about that. But it may be worthwhile to include logic to check for both versions of the file and then use the one that exists. Thoughts?

[fwcd]

Sure, I've updated the logic to include both the new and the old path and choose the first one that exists.

[snoyberg]

Any changes that could be relevant to users have been recorded in the ChangeLog.md (no user-facing changes, so not necessary)

It would actually still be nice to include this in the changelog so that, if someone is hitting this issue on an older version, looking at the changelog will let them know it's fixed.

[fwcd]

The branch is now rebased on master and includes the updated changelog and the suggestion.

[snoyberg]

Thanks!