ci: use uvx to run maturin in native wheel builds

The free-threaded matrix entries skip `uv sync` to avoid resolving
project dependencies against cp313t/cp314t (many dev deps lack
free-threaded wheels), so `uv run --no-project maturin` failed on
macOS/Windows with "Failed to spawn: `maturin`". Switch to
`uvx maturin@1.8.1`, which runs maturin in an isolated tool env
independent of the project venv and matches the pin used by
maturin-action for manylinux builds.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

.github/actions/build-wheel/action.yml

@@ -89,4 +89,6 @@ runs:
     - name: Build via native maturin
       if: inputs.manylinux == ''
       shell: bash
-      run: uv run --no-project maturin build ${{ steps.args.outputs.args }}
+      # Use `uvx` so maturin is available even when `uv sync` was skipped
+      # (free-threaded matrix entries don't pre-populate the project venv).
+      run: uvx maturin@1.8.1 build ${{ steps.args.outputs.args }}