Add debian packaging to CI

[erikjohnston]

We keep breaking the debian package builds, which is annoying when trying to release.

We may want to consider just running this on develop/release branches to save resource?

Presumably we don't want to use the debs built in CI, in case they get compromised?

[anoadragon453]

@richvdh mentioned an alternative which was to build debian packages on RC's as well - which would allow us to catch issues sooner on less important releases.

[richvdh]

not so much an alternative, as something that might be nice to do in addition

[richvdh]

(for links: #6116 is about packaging RCs)

[callahad]

For CI, which Debian / Ubuntu release do we want to build for?

Building for all releases seems slow and wasteful.

The stable releases should be... stable. So I suspect building for testing in CI would be the best way to catch breakage. And in fact, only builds on testing failed for #9073

[richvdh]

counterpoint: we're more likely to accidentally introduce an incompatibility with an old distro.

suggested compromise: build for oldest-supported distro and sid ?

[callahad]

#9073 worked on oldstable, stable, and sid, but not on testing

So let's go with oldest supported + testing rather than sid?

[richvdh]

with the exception of that one datapoint, it feels like working on stable and sid but not testing would be more unusual than it working on testing but not sid, but I don't feel too strongly.