Backport "Use RHJOBS for create-tarball" to fix builds on F38

rpm-4.18.1 recently modified how _smp_mflags is defined and it breaks tarball creation: rpm --eval %{_smp_mflags} because the output is -j${RPM_BUILD_NCPUS} instead of -j8.

While that is being worked on, let's simplify things and use what we already have: RHJOBS.

Now the user controls how many threads with 'make -jX'.

Signed-off-by: @dzickusrh