jobs/seed-github-ci: add openshift/os

The Prow CI we have in those repos are extremely slow and annoying to
maintain. We're still going to need it for now to at least build RHCOS
with actual RHEL RPMs, but at least for CentOS Stream we should be able
to build that fine in CoreOS CI. (We don't have access to the OCP RPMs,
but with openshift/os#799, we'll move those
out of the base compose anyway.)

jobs/seed-github-ci.Jenkinsfile

@@ -16,6 +16,7 @@ repos = [
     "coreos/rpm-ostree",
     "coreos/ssh-key-dir",
     "coreos/zincati",
+    "openshift/os",
     "ostreedev/ostree"
 ]