Fix #23232: Don't use world-writable cache directory anymore.

This pull request is to remove the world-writable cache in /Library/Caches/Homebrew.  The world-writable cache allows a process running under any uid to hijack another user's `brew install` session.

It's still possible to use a world-writable cache by creating it yourself and setting HOMEBREW_CACHE in the environment, but it would no longer be the default behavior.

Closes #23232