Search for exact name in installed cask list

Casks are installed/uninstalled if they are present in the output of `brew cask list`, but this check is done with grep using the cask's name as a pattern. This will give false positives if a cask's name is a subset of another name, such as, for instance, "r". Instead, the search should be on the exact name, anchored with `^` and `$` to avoid substring matching.