Cirrus-CI: Use nproc instead of sysctl -n hw.ncpu

It is available on all supproted FreeBSD versions.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 5bba547a678beb280bf4080375f45b04ceb44fc5)
(cherry picked from commit f283515a1015b5e02dee2b3f4504e7a5ca1a5cfe)