ci: set UV_PYTHON so uv sync keeps the free-threaded interpreter

Pinning only 'uv venv --python' was not enough: 'uv sync' ignores the
existing .venv, runs its own interpreter discovery, and recreated the
venv with the system 3.12, again mismatching the cp313t wheel. Set
UV_PYTHON to the setup-python interpreter for the install and test
steps so every uv command (venv, sync, pip, run) uses it.

Author: timsaucer

.github/workflows/test.yml

@@ -79,12 +79,15 @@ jobs:
           path: wheels/
 
       - name: Install from pre-built wheels
+        env:
+          # Pin every uv command to the interpreter from setup-python.
+          # Without this, `uv sync` ignores the existing .venv, runs its
+          # own discovery, and grabs the system 3.12, producing a
+          # wheel/ABI mismatch on free-threaded jobs.
+          UV_PYTHON: ${{ steps.setup-python.outputs.python-path }}
         run: |
           set -x
-          # Use the interpreter installed by setup-python. Passing a bare
-          # version like "3.13t" lets uv fall back to a different system
-          # interpreter (e.g. 3.12), producing a wheel/ABI mismatch.
-          uv venv --python "${{ steps.setup-python.outputs.python-path }}"
+          uv venv
           uv sync --dev --no-install-package datafusion
           WHEELS=$(find wheels/ -name "*.whl")
           if [ -n "$WHEELS" ]; then
@@ -99,6 +102,7 @@ jobs:
       - name: Run tests
         env:
           RUST_BACKTRACE: 1
+          UV_PYTHON: ${{ steps.setup-python.outputs.python-path }}
           # On free-threaded interpreters, fail loud if any C extension
           # re-enables the GIL implicitly.
           PYTHON_GIL: ${{ matrix.freethreaded && '0' || '' }}