[llvm] [ci] limit parallel windows compile jobs to 24 (PR #93329)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> I collected statistics some time ago: https://discourse.llvm.org/t/build-time-comparision/56024 (including for `/cgthreds1`). Usage may have grown since then.
> 
> According to the [Discourse post](https://discourse.llvm.org/t/flang-tests-are-extremely-slow-on-windows/78591/11?u=meinersbur), there is a 20 min wait where nothing happens. This matches the `--timeout=1200` LIT parameter. That is, there may be a deadlock in some test that is eventually given up on by LIT.

FWIW, I see this from a clangd test with some regularity:
```
timeout=1200 --time-tests C:/ws/src/build/tools/clang/tools/extra/include-cleaner/test C:/ws/src/build/tools/clang/tools/extra/pseudo/test C:/ws/src/build/tools/clang/tools/extra/clangd/test/../unittests C:/ws/src/build/tools/clang/tools/extra/clangd/test C:/ws/src/build/tools/clang/tools/extra/test"
```
from https://buildkite.com/llvm-project/github-pull-requests/builds/65934

https://github.com/llvm/llvm-project/pull/93329