darwin: Make nproc run in the docker instance

Darwin doesn't have an nproc command, so unless we run this in the docker
container, we won't be able to see how many cores we have. Additionally, this
fixes setup.sh to skip the inclusion of the j command, since Darwin doesn't run
a recent bash instance.

Change-Id: Ia6423a2da7500fb8337b05bc32fcc901a0ff247b
2 files changed