Bring CI back to all-working condition (#5822)

* Fix "🐍 3 • windows-latest • mingw64" job (apparently msys2/setup-msys2@v2 cannot be run twice anymore):

https://github.com/pybind/pybind11/actions/runs/17394902023/job/49417376616?pr=5796

```
Run msys2/setup-msys2@v2
  with:
    msystem: mingw64
    install: mingw-w64-x86_64-python-numpy mingw-w64-x86_64-python-scipy mingw-w64-x86_64-eigen3
    path-type: minimal
    update: false
    pacboy: false
    release: true
    location: RUNNER_TEMP
    platform-check-severity: fatal
    cache: true
  env:
    PYTHONDEVMODE: 1
    PIP_BREAK_SYSTEM_PACKAGES: 1
    PIP_ONLY_BINARY: numpy
    FORCE_COLOR: 3
    PYTEST_TIMEOUT: 300
    VERBOSE: 1
    CMAKE_COLOR_DIAGNOSTICS: 1
    MSYSTEM: MINGW64
Error: Trying to install MSYS2 to D:\a\_temp\msys64 but that already exists, cannot continue.
```

* Add `pytest.xfail("[TEST-GIL-SCOPED] macOS free-threading...)`

* Change env.SYS_IS_GIL_ENABLED constant to env.sys_is_gil_enabled function

* Change install_mingw64_only → extra_install

* Also xfail if macOS and PY_GIL_DISABLED, show SOABI

* build-ios: brew upgrade|install cmake

* Revert "build-ios: brew upgrade|install cmake"

This reverts commit bd3900ee794a01bb1ef91a9355ce72be619a7196.

See also:

https://github.com/pybind/pybind11/pull/5822#issuecomment-3247827317

* Disable build-ios job in tests-cibw.yml

* Remove macos_brew_install_llvm job because it started failing, to reduce our maintenance overhead:

Failures tracked here: https://github.com/pybind/pybind11/pull/5822#issuecomment-3247998220

* Fix iOS build step for cmake installation

Replaced brew upgrade with brew install for cmake.

* Update cmake installation steps in CI workflow

Uninstall cmake before installing the latest version due to GitHub's local tap changes.

* Update .github/workflows/tests-cibw.yml

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Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>