[workflow] Use MSVC problem matcher for Windows action build by ammaraskar · Pull Request #18532 · python/cpython

It's a little unfortunate that all of the warnings are duplicated (or triplicated, in some cases).

Yeah, looks like MSVC outputs the warnings on the files they occur and then a summary leading to the duplication. VSCode de-duplicates warnings if the text and line numbers match exactly but Github Actions don't. I've filed actions/runner#504 upstream, let's see what they say.

do we know for sure what will happen there? Is it every warning, or just new ones that appear?

The behavior is that if you make changes to a file without the warnings, then they will appear like they do in this PR. At the bottom and marked as Unchanged files with check annotations. If you touch a file that had existing warnings then they'll show up in the actual diff.

As such, before merging this we should try to get rid of as many as the warnings first I think.