Changes

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 https://github.com/openshift/os/issues/799, we'll move those
out of the base compose anyway.)

jobs/seed-github-ci: fix trigger job DSL definition

Looks like this might've changed at some point in a recent bump of the
job DSL plugin.

jobs/seed-github-ci: fix cron() argument type

I thought the int there would automatically get coerced to a string, but
that's not the case.

jobs/seed-github-ci: fix branch rescan trigger

I think I got it right this time. The docs for that key specifically
mention SCM re-indexing and push notification fallback.