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

[Nikolas Klauser via llvm-commits]

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@@ -199,3 +199,44 @@ jobs:
             **/CMakeError.log
             **/CMakeOutput.log
             **/crash_diagnostics/*
+  windows:
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philnik777 wrote:

I'd rather only wait for stage1, since the GitHub-provided agents are quite slow. I think it's quite unlikely that we'll actually hit resource limits, since we have up to 60 concurrent agents available, i.e. more than 7 libc++ patches uploaded in an hour that don't fail on stage1. We can still put that higher later if we do hit limitations at some point.

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