qt: fix superenv and other cleanup.
A more helpful error message for `brew untap`

`brew untap` without arguments currently errors out (as it should), but the
error message is generic and unhelpful. (The error message is borrowed from
a method in the tap command - `tap_args`.)

Closes #18186.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>