Fix ZDOTDIR environment variable not being read

[leoheitmannruiz]

• 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?

---

% echo $ZDOTDIR
/Users/leo/.config/zsh
% brew info openjdk | grep echo
  echo 'export PATH="/opt/homebrew/opt/openjdk/bin:$PATH"' >> ~/.zshrc

ZDOTDIR is supposed to be read, but as it's not on the "allow list", it isn't. At least, that's my understanding.

I just copied what has been done for TMUX:

% grep -nr TMUX
brew/bin/brew:180:  TMUX
brew/Library/Homebrew/caveats.rb:186:    if ENV["HOMEBREW_TMUX"] && !quiet_system("/usr/bin/pbpaste")

But I can also instead add it to ENV_VAR_NAMES, if that is desired.

[MikeMcQuaid]

Looks great, thanks @leoheitmannruiz! One test looks like it needs fixed here but good to 🚢 when 🟢