use OpenMP as numba threading layer on MacOS

[AgnieszkaZaba]

MacOS needs OpenMP threading layer installed using this setup:

os.environ['NUMBA_THREADING_LAYER'] = 'omp'

The error is in this run: https://github.com/open-atmos/PySDM/actions/runs/19776472272/job/56670051527?pr=1748#step:6:1169

[slayoo]

due to #1183, we don't use threading at all on macOS