During CI, use nproc instead of sysctl -n hw.ncpu

The latter may give the wrong result if cpusets are in use.

Sponsored by: ConnectWise
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #18012