PIPE-2926: CI/CD workarounds
- Target macOS 15+ in pixi system-requirements
Set macos = "15.0" so pixi prefers the latest `casatools``
wheels on `osx-arm64``. `uv`` may still fall back to a 14.0
wheel when no 15.0 build is available.
- Update `Pixi` setup action to version `0.9.4` and adjust `Pixi`` version to `0.62.2`
- Disable `pytest-xdist` for parallel workers with '-n 0' to avoid
resource contention on the GitHub macOS runners.
