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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Ah, didn't realize the Github changes were that recent.

Providing everything in the base image definitely makes sense. More things probably need to be reworked either way to get things working on new machines and it's not like this moves the needle at all.

Thanks for the info.

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