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

[Tom Stellard via llvm-commits]

tstellar wrote:

Can you limit the total number of jobs to 40?  We need to have some spare capacity for running some of the other jobs, like the PR labeler, code_formatter, etc.  We may be able to bump it higher in the future, but I think 40 is a good start.

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