Pass the list of choices to argparse.Action

Pass the list of choices to argparse.Action

This way our custom ListAction action can properly generate a list of
choices in the help message when the metavar is not defined:

Before:
$ mkosi --help
...
  --debug SELECTOR      Turn on debugging output

After:
$ mkosi --help
...
  --debug {run,build-script,workspace-command}
                          Turn on debugging output