Fix TMUX env variable filtering

[carlocab]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

We currently filter out TMUX, but this breaks displaying some caveats. This also enables an alias I use (improved by @Rylan12 -- thanks Rylan):

brew alias fzp='!id="$(gh pr list -L200 -R github.com/Homebrew/homebrew-core | TMUX=$HOMEBREW_TMUX fzf-tmux -p "90%,50%" --multi | cut -f1)"; [ -n "$id" ] && brew pr-publish --autosquash $id'

Alternatively, we could just stop filtering TMUX entirely. I've patched bin/brew locally to do that, but I don't know how we feel about adding more env variables to our whitelist. In particular, TMUX was mentioned in #1753 but never added, so I assume there's a reason for that.

Or, if we don't want to define HOMEBREW_TMUX, we should just remove the caveat that relies on it.

[BrewTestBot]

Review period will end on 2021-05-14 at 16:14:58 UTC.

[Rylan12]

Ooh, forgot that we can map TMUX directly to HOMEBREW_TMUX! Even better!

[BrewTestBot]

Review period ended.