Don't treat non-directory arguments as paths.

Library/Homebrew/cask/config.rb

@@ -54,7 +54,15 @@ module Cask
     end
 
     def self.canonicalize(config)
-      config.map { |k, v| [k.to_sym, Pathname(v).expand_path] }.to_h
+      config.map do |k, v|
+        key = k.to_sym
+
+        if DEFAULT_DIRS.key?(key)
+          [key, Pathname(v).expand_path]
+        else
+          [key, v]
+        end
+      end.to_h
     end
 
     attr_accessor :explicit