pass: use any gpgtools.

The homebrew-provided version of GPG is quite difficult to configure,
and Homebrew Cask provides a nice option to install GPGTools (which
takes care of a lot of configuration). This changes the pass formula to
skip installing the Homebrew gnupg2 package if gpg is already installed
through some other method (most likely GPGTools).

Closes #33684.

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