Use COMPOSE_ID to fetch the correct, latest working image

[cjeanner]

In order to avoid weird issues, it would be nice to take this into account:
http://mirror.stream.centos.org/9-stream/COMPOSE_ID

At this URI, we have the latest known-to-work image name. It will require some regex and work to extract the "date" and build the actual image name in order to download it, but at least, it will ensure we're able to deploy without hitting some weird package issue such as https://launchpad.net/bugs/1964477