Skip tapping homebrew/cask when on Linux
Skip tapping homebrew/cask when on Linux

- Casks are automatically skipped on Linux. When there's a Brewfile used
  on both macOS and Linux, there's usually Casks in it. This leads to
  `tap "homebrew/cask"` being present and causes the Cask tap to be
  installed on Linux too, even though Casks don't work on Linux and
  `cask` entries in Brewfiles are auto-skipped.
- A recent side effect of this behaviour is that because `brew update`
  now shows Cask data too, Linux users who cannot make use of Casks are
  see a lot of noise, and may get their hopes up that Casks are
  available.
- Alternatives to this feature included:
    - Putting up with it.
    - Skipping outputting `brew update` info for Casks if on Linux.
    - Adding `tap "homebrew/cask" if OS.mac?` to my Brewfiles.

Homebrew/homebrew-bundle