databases/mongodb70: make_jobs_unsafe

For unknown reasons this port tends to fail a lot on @armv80a flavor.

From the build log:
"scons: *** [build/opt/mongo/db/catalog/collection_uuid_mismatch.o] Error -9"

I can't reproduce this locally. Let's try to remove parallel jobs and
see if it improves the build stability.

Reported-by: https://portsfallout.com/fallout?port=databases%2Fmongodb%5B0-9%5D&maintainer=&env=&category=&flavor=