Previously, the actions were duplicated, due to the fact when developing
we often create feature branches and open pull requests.
This new setup requires us to open pull requests to trigger the CI.
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.
