Improved Debian package CI

Description

The Debian package CI building process can be improved. Originally discussed here.

Tasks

• Fix the CI for bookworm. Currently it's failing to find the lintian executable, despite the package being installed (example job run). Task moved to #105 (closed).
• Create a CI job for Debian unstable.
• Fix issue with the piuparts job, were sometimes (depending on the runner?) it fails to mount something (example success, example failure):
  • Create a Minimum Reproducible Example (MRE). Done at https://gitlab.torproject.org/rhatto/piupartisan.
  • Apply a workaround. Done on !97 (merged).
  • Report the bug upstream. Done at tpo/tpa/team#42087 (closed).
• Add a sample autopkgtest job. Initial work done at !98 (merged), but since there are no tests, the job was disabled.
• Consider using sbuild (docs) for building the debian package, as it outputs potentially useful information (example) that could prevent issues like 1070077. Note that this might be a bit overkill since we're already in a sort of a chroot (a container), so we might also consider a lightweight alternative. Initial work done at !98 (merged), but the job is disabled due to many chroot errors. Maybe sbuild is too much for (rootless) containers.
• Cleanup/merge Debian provision procedures: there might be duplicate/unneeded code/packages in both scripts/provision-debian-packaging and in the CI configuration. Done on 9061b6a1 and 64f69e40.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)