Switch macOS CI back to MPICH once upstream ch4:ofi finalize bug is fixed

[hughcars]

We changed the macOS matrix entry clang/mpich/openblas/static/int32/openmp/slepc/strumpack from MPICH to OpenMPI to dodge an upstream MPICH bug: on macOS, ch4:ofi aborts in MPI_Finalize (MPIDI_OFI_handle_cq_error: OFI poll failed) even though all test assertions pass. It's unfixed across MPICH 4.1–5.0.1 (pmodels/mpich#6856, pmodels/mpich#7760) and only avoided via FI_PROVIDER=tcp, an old 4.0.2, or the ch3 device.

MPICH on macOS is still covered (serial) by the other entry; only MPICH+OpenMP coverage is dropped.

Revert to mpi: mpich once MPICH fixes it and Homebrew ships the fix.