[llvm] [libcxx] [libc++] Use GitHub-provided runners for the windows CI (PR #79326)

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

TBH I think this LGTM if the CI passes.

I do expect that we'll hit resource limitations pretty quickly, but I guess we can cross that bridge when we get to it. This might entail setting up self-hosted runners for Windows.

https://github.com/llvm/llvm-project/pull/79326