pyarrow tests are executed twice in py38-pyarrow-x tox task

tox -e py37-pyarrow-2 runs all tests with the "uses_pyarrow" mark twice. This is because run_pytest.sh runs pytest twice, once with -m no_xdist and once with -m 'not no_xdist'. The pyarrow task uses -m uses_pyarrow to select tests, but pytest only respects the latest -m argument. So the no_xdist parameters are ignored, and all pyarrow tests are run each time.