Currently, run-pytest.sh won't run locally outside of a pipeline
because it can't find the `setup-test-env.sh` which provides necessary
functions.
Add a default value for the SCRIPTS_DIR so that run-pytest can find and
run the setup-test-env.sh and be run locally outside of a pipeline.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32602>