Migrate workbench to use Podman Toolbox.

- wb shell function to launch tmux inside a toolbox container locally.
- wbr shell function to do the same but on ns4.
- Remove the workbench SSH config, not used now.
- Set the DOCKER_HOST environment variable to the Podman socket in case
  the Docker socket isn't available but the Podman socket is.
- A few aliases (for now flatpak-spawn and podman) that are set if
  running in a toolbox container.