gitlab-ci.d/custom-runners: Improve rules for the staging branch

gitlab-ci.d/custom-runners: Improve rules for the staging branch

If maintainers are currently pushing to a branch called "staging"
in their repository, they are ending up with some stuck jobs - unless
they have a s390x CI runner machine available. That's ugly, we should
make sure that the related jobs are really only started if such a
runner is available. So let's only run these jobs if it's the
"staging" branch of the main repository of the QEMU project (where
we can be sure that the s390x runner is available), or if the user
explicitly set a S390X_RUNNER_AVAILABLE variable in their CI configs
to declare that they have such a runner available, too.

Fixes: 4799c21023 ("Jobs based on custom runners: add job definitions ...")
Message-Id: <20210728173857.497523-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>