macOS nightly: two cores
macOS nightly: two cores

All GitHub actions provide two cores. Just make sure this does not
get oversubscribed with potentially visible additional physical
cores.

LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack