Enable parallel compilation for FreeBSD QCOW2 jobs
Packer jobs should also benefit from parallel compilation the way Docker jobs do. This only applies to FreeBSD jobs for the time being (since we do not compile any source code while building other QCOW2 images), but make the necessary adjustments on a "global" level (in packer/Makefile) anyway, so that further platforms can be tweaked more conveniently if the need arises in the future. While packer/freebsd/packer.json only allocates 2 vCPUs to the VM building the QCOW2 image and the default number of parallel build jobs is set to 6, we do not care much about that discrepancy.
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