Add 'HOMEBREW_DEBUG_PREFIX'

When doing "brew install -i", set the "HOMEBREW_DEBUG_PREFIX"
variable to the prefix of the formula being installed.

Library/Homebrew/install.rb

@@ -85,6 +85,7 @@ def install f
           puts "to copy the diff to the clipboard."
         end
 
+        ENV['HOMEBREW_DEBUG_PREFIX'] = f.prefix
         ENV['HOMEBREW_DEBUG_INSTALL'] = f.name
         interactive_shell
         nil