CI: Use PYTEST_WORKERS=0 instead of 1 for single cpu runtime
It appears `-n 1` with pytest-xdist will spawn a worker and run a test on the single worker while `-n 0` avoids that extra work of spawning a worker and still runs the tests in a "single cpu" runtime
Looks like this shaves off significant time off the windows builds which is nice. Going to merge but happy to follow up.