Only serialize native-deb* targets

.NOTPARALLEL target is being forced on userspace as well. This commit
removes .NOTPARALEL target and only serializes the execution of
native-deb* targets.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #16622