Adjust `brew cask doctor` for homebrew-fork

Since #8089 we are pulling in code from an incorporated Homebrew fork.

This means that some of the output of `brew cask doctor` was deceptive.  After this PR, we no longer report `HOMEBREW_*` constants derived from our fork, but read them at runtime by executing *eg* `brew --cellar`.

This also removes a direct dependency on the fork.

It is debatable whether we should continue to provide this info in "doctor", as it no longer has the same degree of influence on the execution of `brew cask`.  But it does no harm, and we can revisit it later.