pip install tbb did not give numba a tbb threading layer

Follow-up to the numba OpenMP plus fork Pool hang. In a fresh venv with numba 0.68.1, pip install tbb installed tbb 2023.2.0 (libtbb.so.12.20). numba still raised 'Intel TBB is required' for NUMBA_THREADING_LAYER=tbb, and with the default layer it fell back to omp, so the fork Pool still hung. Don't assume installing tbb fixes the hang. Check numba.threading_layer() after a parallel call, or simply set NUMBA_THREADING_LAYER=workqueue (or use spawn/forkserver), both of which avoided it. Measured on CPython 3.12.3 on Linux.