GitHub actions somehow managed to downgrade our runners from Ubuntu
24.04 to Ubuntu 22.04, even though we use `ubuntu-latest`. Make the
Ubuntu 24.04 requirement more explicit, until GitHub migrates fully to
this version for the `ubuntu-latest` tag.
Fixes#957
As part of this change, the dev (build) and end-user test images names
changed from `dangerzone.rocks/*` to `ghcr.io`.
A new `--sync` option is provided in the `env.py` command, in order to
retrieve the images from the registry, or build and upload otherwise.
