Don't attempt to cleanup source checkouts

Fixes Homebrew/homebrew#10751.

Library/Homebrew/cmd/cleanup.rb

@@ -50,6 +50,7 @@ module Homebrew extend self
 
   def clean_cache
     HOMEBREW_CACHE.children.each do |pn|
+      next unless pn.file?
       pn.stem =~ /^(.+)-(.+)$/ # greedy so works even if formula-name has hyphens in it
       if $1 and $2
         f = Formula.factory($1) rescue nil