Bootstrap images with mkosi

[spanezz]

mkosi can bootstrap OS images in a way that works with Moncic-CI, and we could just delegate bootstrapping to it.

Unfortunately, moncic-ci seems currently better at bootstrapping than mkosi. See for example:

• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1008169 (cannot bootstrap rpm distributions from Debian)
• bootstrapping a CentOS 7 results in unusable YUM and no rpm db systemd/mkosi#934 (cannot bootstrap some old rpm distributions from newer rpm distributions)

We can still use mkosi as a default bootstrap method for all distributions for which we do not have an explicit better way of doing it, and we can consider our explicit ways as workarounds for mkosi issues, that will hopefully go away as mkosi improves

[bluca]

The mkosi issue is due to a Debian-specific RPM patch, and it's fixed in main and Debian unstable (and testing in a few days).