- Without `DISPLAY` as an envvar in `bin/brew`, running `brew edit`
  with $EDITOR set to a graphical editor (eg `gedit`), it errored on
  Linux:

```
$ EDITOR=gedit brew edit vim

Editing
/home/linuxbrew/.linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/vim.rb
Unable to init server: Could not connect: Connection refused

(org.gnome.gedit:15470): Gtk-WARNING **: 18:17:07.537: cannot open display:
Error: Failure while executing; `gedit
/home/linuxbrew/.linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/vim.rb`
exited with 1.
```

Fixes #6958.