gitlab-ci: Only use gstreamer runners for the s390x job for now
The fdo-packet-* runners keep hitting the (already quite long) timeouts for some of the tests, taking many times as long for them as the gstreamer runners.
The fdo-gitlab-gce-runner3 runner would work as well, but it doesn't have any tags we could use.
/cc @daniels
Edited by Michel Dänzer