[ci] Cover all supported Pythons and both LLVM majors per PR - #78

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-python@v5, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

crash-class markers passed in isolation; see the job summary

llvm@21 was installed but not linked because llvm@20 is already installed. To link this version, run: brew link llvm@21

flavor=system source=package-manager prefix=/Users/runner/work/cppjit/cppjit/install