Add hotfixes-rhel-coreos-9.yaml

[cgwalters]

This builds on coreos/coreos-assembler#3352 to add the latest build of RHEL8 rpm-ostree inside our extensions container so the MCD can live-apply it.

Part of https://issues.redhat.com/browse/OCPBUGS-7275

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[travier]

8.6 here but you mention 8.8? Isn't this RPM more likely to be a custom rhaos build instead?

[cgwalters]

Yeah, this is just a demonstration change. That said, I do think it'd likely make sense to get the change directly into 8.6.z too.

[cgwalters]

OK as of right now, we changed plan and are shipping the fixed rpm-ostree back to 4.12. But, at least we have this technology now in case we need it in the future.