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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by:	Alan Somers <[email protected]>
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"