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

[mkoeppe]

as suggested in #35652 (comment)

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