Build systemd-sysupdate compatible images

if we want to use systemd-sysupdate for updates we need to build images that the service can pull and host them somewhere. It probably makes sense to do then as part of the monthly stable build workflow here so that the update images stay in sync with the OS images I think?

It looks like those should be built with systemd/mkosi

more info here: https://mkosi.systemd.io/