cleanup: fix stale cask detection.

This method is also run to check formulae and if a formula and cask have
the same name (e.g. `cmake`) then this method would mark all formulae
downloads to being stale.

Instead, check the `dirname` (which we're using for globs anyway) to
double check that this is definitely a stale cask.

Library/Homebrew/cleanup.rb

@@ -112,6 +112,8 @@ module CleanupRefinement
     def stale_cask?(scrub)
       return false unless name = basename.to_s[/\A(.*?)\-\-/, 1]
 
+      return if dirname.basename.to_s != "Cask"
+
       cask = begin
         Cask::CaskLoader.load(name)
       rescue Cask::CaskUnavailableError