fix stylesheet cache to recover if file is on disk

lib/sass/discourse_stylesheets.rb

@@ -118,10 +118,14 @@ class DiscourseStylesheets
     end
   end
 
-  def cache_fullpath
+  def self.cache_fullpath
     "#{Rails.root}/#{CACHE_PATH}"
   end
 
+  def cache_fullpath
+    self.class.cache_fullpath
+  end
+
   def stylesheet_fullpath
     "#{cache_fullpath}/#{stylesheet_filename}"
   end