Extend CI to Ubuntu

The tooling for running CI against Ubuntu is basically identical. Basically, some options in debci.conf need to be changed.

Since the existing workers run the tests in QEMU VMs, the same physical workers could be used for the Ubuntu CI, provided that the worker is started with an alternative debci config dir. This can be done manually already, but @ckk is working on a solution to automate this.

At first glance, Debian and Ubuntu CIs would need separate databases for job and results management. It would be neat if we'd have a single page where test results from both CIs are aggregated.

The major obstacle to implementing this right away is that debootstrap currently has no scripts for creating recent Ubuntu releases, the most recent script is for focal.

Additionally, this depends on debci #5.