[gitian-build] Default the number of jobs to the number of CPUs

Summary: We already do this on TeamCity, and there's no reason not to do this here.

Test Plan:

gitian-build.py -h

# In VM
./bitcoin-abc/contrib/gitian-build.py -c -n -b -o m -m 3500 gitianuser v0.19.10

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3687