Use HOMEBREW_REPOSITORY for freshness check

Homebrew itself is found under HOMEBREW_REPOSITORY,
which *may* be the same HOMEBREW_PREFIX, but does not have
to be if you link brew into /usr/local/bin from another location.

Library/Homebrew/cmd/doctor.rb

@@ -801,11 +801,11 @@ def check_for_bad_python_symlink
 end
 
 def check_for_outdated_homebrew
-  HOMEBREW_PREFIX.cd do
+  HOMEBREW_REPOSITORY.cd do
     timestamp = if File.directory? ".git"
       `git log -1 --format="%ct" HEAD`.to_i
     else
-      (HOMEBREW_PREFIX/"Library").mtime.to_i
+      (HOMEBREW_REPOSITORY/"Library").mtime.to_i
     end
 
     if Time.now.to_i - timestamp > 60 * 60 * 24 then <<-EOS.undent