[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.

I thought this was possible, but I haven't found a way to actually limit the number of jobs.  I f you know how to do implement this then that is great, but otherwise do you have an estimate of the number of concurrent machines you will need to handle the peak load?

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