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.
Acked-by: Daniel Stone <daniels@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3760>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3760>