ci: run tests from the .venv, not the bare setup-python interpreter

Setting UV_PYTHON on the test step pointed 'uv run --no-project pytest'
at the setup-python interpreter, which has no pytest installed, causing
'Failed to spawn: pytest'. UV_PYTHON is only needed in the install step
to build the .venv with the right interpreter; the test step must use
that .venv. Drop UV_PYTHON from the test step.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: timsaucer

.github/workflows/test.yml

@@ -102,7 +102,6 @@ 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' || '' }}