Validate snap building process

[tonyandrewmeyer]

We've had some issues with the snap remote-build recently. I think it would be worth checking whether we are doing this according to current best practice. remote-build says it's experimental, is that important? snapcrafters/ci has some actions - we don't really like adding dependencies, but maybe it would be worth using them, or copying techniques? Maybe we should use the hosted runners and so local builds? Our system feels old and clunky, so it would be nice to improve that.

[hpidcock]

Since we are not building any exotic architectures at the moment, lxd building should be sufficient on Canonical runners.