ci: specify number of jobs when executing `make`

Unlimited number of make jobs may lead to OOM. We should use
`make -j $(nproc)` instead of `make -j` on Linux. Note, on OSX
and FreeBSD the `nproc` command is unavailable out of the box.
So using `sysctl -n hw.ncpu` instead.

Closes #7020

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci