buildstream/_project.py: Restrict max-jobs

Description

Restrict max-jobs even if the machine have mores cores available

This will mitigate situations on where a big machine (96 cores) executing bst on docker containers will try to execute all of those instances with -j96, making everything slower overall (I'm personally managing 3 of this machines at the moment for different projects)

Patch taken from @devcurmudgeon from a previous analysis with another build tool (YBD): baserock/ybd@580b7da0