fix .xz in stdenv bootstrap

Presumably needed due to the last staging merge.
There was some stdenv stage refactoring within, IIRC.

pkgs/tools/text/diffutils/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, coreutils ? null }:
+{ stdenv, fetchurl, xz, coreutils ? null }:
 
 stdenv.mkDerivation rec {
   name = "diffutils-3.3";
@@ -11,7 +11,7 @@ stdenv.mkDerivation rec {
   outputs = [ "out" "info" ];
 
   /* If no explicit coreutils is given, use the one from stdenv. */
-  nativeBuildInputs = [ coreutils ];
+  nativeBuildInputs = [ xz.bin coreutils ];
 
   meta = {
     homepage = http://www.gnu.org/software/diffutils/diffutils.html;