[win][ci] Update LLVM toolchain used to build LLVM to 20
While trying to get the aarch64-msvc build working correctly (#140136), I needed to update the version of LLVM used to build LLVM in Windows CI runners to 20 (as this has improved support for Arm64 and Arm64EC on Windows).
This catches Windows up to Linux which was updated to 20 by #137189
try-job: `x86_64-apple-*`
try-job: `aarch64-apple`
try-job: `x86_64-msvc-*`
try-job: `i686-msvc-*`
try-job: `x86_64-mingw-*`
CI: Stop /msys64/bin from being prepended to PATH in msys2 shell
We used to do this along time ago but we stopped doing it when we started installing msys2 manually. 4fd3cf96a1/src/ci/scripts/install-msys2.sh (L11-L13)Fixes#136795
try-job: dist-i686-mingw
[CI] Upload average CPU utilization of CI jobs to DataDog
This PR adds a new CI step that uploads the average CPU utilization of the current GH job to Datadog. I want to add more metrics in follow-up PRs.
r? `@jdno`
try-job: dist-i686-msvc
try-job: aarch64-apple
try-job: x86_64-gnu-llvm-18
This partially reverts commit fe7c97c2e7.
I kept a mv, not a cp, for the one that shuffles major artifacts around,
because the size of those artifacts are big enough to matter, sometimes.
I don't think the diagnostic info will be that heavy, by comparison.
CI: add a script for dynamically computing CI job matrix
It would be great if was easier to run specific CI workflows locally, and also to allow us to spawn a specific CI workflow by bors, to enable running arbitrary try builds. See discussion [here](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/CI.20workflows.20refactoring).
This PR is a first step in that direction.
- Moves the definition of CI runners and (for now) PR jobs into a separate `jobs.yml` file.
- Adds a simple Python script that reads the file, decides which jobs should be active for the current CI workflow, and prints them as JSON to their output.
- The PR job then reads this output and generates its job matrix based on it.
By moving the job definitions from `ci.yml` into a separate file, we can handle it programmatically, which should make it easier to both do local execution of CI jobs and also to do arbitrary try builds.
Errors started showing up, and I read somewhere that this might be
because of old ninja versions. This ninja version is indeed *ancient*.
```
multiple outputs aren't (yet?) supported by depslog; bring this up on the mailing list if it affects you
```
