style: resolve symblic links in HOMEBREW_CACHE

[notfirefox]

On Linux HOMEBREW_CACHE may be set to $HOME/.cache/Homebrew. Some systems, like Fedora Silverblue, have /home set up to be a symbolic link to /var/home. While Homebrew generally seems to work fine on Fedora Silverblue, rubocop causes the brew style command to emit a lot of warnings because of the symbolic link[1]. This can be resolved by looking up the real path of HOMEBREW_CACHE before passing it to rubocop.

[1] rubocop/rubocop#6228

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[MikeMcQuaid]

Good idea, thanks @notfirefox!