[llvm] [libcxx] [ci] Stop manually installing ninja in the Windows build jobs (PR #161907)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> This might be left over from before the libc++ CI was running inside Github hosted runners?

No, this used to be necessary; there was a recent change on github hosted runners to provide ninja pre-installed - see https://github.com/actions/runner-images/pull/11737 and https://github.com/actions/partner-runner-images/issues/96.

> I guess this makes the workflow less portable if someone wanted to try getting it running again on more powerful windows machines, but given the instability, this seems like a decent enough option for now.

I guess that's true, but depending on who's providing those more powerful runners, I would kinda expect ninja to be preinstalled anyway - ideally all the necessary tools would be preinstalled in the base image.

https://github.com/llvm/llvm-project/pull/161907