In `i3 4.16`, `i3-nagbar` introduces the flags `-B/--button-no-terminal`
to run the action directly instead of inside a terminal. This implements
the flags for swaynag for compatibility.
Since swaynag does not use an equivalent to `i3-sensible-terminal`, the
flags `-b/--button` only uses a terminal when the environment variable
`TERMINAL` is set, otherwise it acts the same as these new flags.
