macOS nightly: two cores

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

.github/workflows/install_spack.sh

 #!/usr/bin/env sh
 git clone https://github.com/spack/spack.git
+echo -e "config:\n  build_jobs: 2" > spack/etc/spack/config.yaml
 . spack/share/spack/setup-env.sh
 spack compilers