Toolbx refresh.

- Debian testing and the next Ubuntu LTS (jammy) have Toolbx packaged.
  Use that instead of the binary from toolbox-build. Keep a list of
releases that don't have that it packaged since it should be pretty
static and over time can be removed.
- Instead of checking the distro release, check if there's a Kubic APT
  repo defined (like the assertion does). So we have a single place to
list releases that need that repo. Since the current Debian stable
(bullseye) and the next Ubuntu LTS (jammy) have Podman packaged that
list should be very stable (unless there's another ElementaryOS release
based on Ubuntu Focal before 7.0).
- Rename Toolbox to Toolbx in task names (match the renamed project
  name).