emacs: fix priority for ctags

This fixes the previous attempt (in commit 46d1dd5) to make ctags' ctags
override emacs' ctags. The higher the value of the priority attribute,
the higher the priority (see the definition of `buildEnv`).

pkgs/applications/editors/emacs-24/default.nix

···
     maintainers = with maintainers; [ chaoflow lovek323 simons the-kenny jwiegley ];
     platforms   = platforms.all;
 
-    # So that Exuberant ctags is preferred
-    priority = 1;
-
     longDescription = ''
       GNU Emacs is an extensible, customizable text editor—and more.  At its
       core is an interpreter for Emacs Lisp, a dialect of the Lisp

pkgs/development/tools/misc/ctags/default.nix

···
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ simons ];
     platforms = platforms.unix;
+
+    # So that Exuberant ctags is preferred over emacs's ctags
+    priority = 1;
   };
 
 }