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

.github/workflows/scripts/qemu-4-build-vm.sh

@@ -181,7 +181,7 @@ function freebsd() {
   echo "##[endgroup]"
 
   echo "##[group]Build"
-  run gmake -j$(sysctl -n hw.ncpu)
+  run gmake -j$(nproc)
   echo "##[endgroup]"
 
   echo "##[group]Install"