fix .xz in stdenv bootstrap

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

pkgs/tools/compression/gzip/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl }:
+{ stdenv, fetchurl, xz }:
 
 stdenv.mkDerivation rec {
   name = "gzip-1.6";
@@ -10,6 +10,8 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  buildInputs = [ xz.bin ];
+
   # In stdenv-linux, prevent a dependency on bootstrap-tools.
   makeFlags = "SHELL=/bin/sh GREP=grep";