.gitlab-ci.yml: keep samba-ci-private tag only for private jobs

.gitlab-ci.yml: keep samba-ci-private tag only for private jobs

This will help us give the legacy 'private' tag, used in branches
under maintenance, more resources without those jobs running on the
normal production runners (therefore avoiding the additional cost for
the 90% of builds that are for master).

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>