We make use of uv to simplify the pipeline in several ways:

- replace 'python -m pip' with 'uv pip'
- set cache to match uv usage
- use uv for CIBW_BUILD_FRONTEND
- set UV_EXTRA_INDEX_URL to our GitLab registry, with
  UV_INDEX_STRATEGY=unsafe-best-match and UV_ONLY_BINARY=:all: so we
  pick whichever source has a binary wheel
- drop 'allow-prereleases' from setup-uv since this only applies to the
  setup-python action

Also make sure we explicitly skip musllinux builds, since upstream isn't
building them either. Otherwise, we run into issues with uv setup:

|Building cp314-musllinux_riscv64 wheel
|CPython 3.14 musllinux riscv64
|
|Setting up build environment...
|
|      + mkdir -p /
|      + /opt/python/cp39-cp39/bin/python -c 'import sys, json, os; json.dump(os.environ.copy(), sys.stdout)'
|      + which python
|      + which uv
|                                                              ✕ 4.07s
|Error: cibuildwheel: Command ['which', 'uv'] failed with code 1.
|
|Error: Process completed with exit code 1.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>