build.yml: Report failures (e.g. from incremental test) earlier

as suggested in https://github.com/sagemath/sage/pull/35652#issuecomment-1603365433

possibly using https://github.com/LouisBrunner/checks-action

and/or problem matchers for:
- sphinx (see https://github.com/python/cpython/pull/20325/files)
- linter errors
- doctest failures
