[rpmbuild] make_srpm: don't use --private-users=pick
Even though --private-users=pick **is** the "recommended" settings
(per man systemd-nspawn), let's use the default
(--private-users=true) setup for user namespaces.
First, this is needed at least till  is merged, and/or
systemd-nspawn is fixed to behave differently.
Second, there's no point to treat make_srpm differently from other
srpm methods, or even from RPM building. If the security was a
concern here, we should use '--private-users=pick' everywhere in
mock by default. Or ideally we shouldn't even grant the user the
root access inside the container at all (might be done in #534).