make-system-tarball.sh: use numeric uids

Ownership should now always be 0/0.

Pointed out by @dezgeg at
https://github.com/NixOS/nixpkgs/pull/21305#discussion_r93258149

Changed files
+1 -1
nixos
+1 -1
nixos/lib/make-system-tarball.sh
···
rm env-vars
-
tar --sort=name --mtime='1970-01-01' -cvJf $out/tarball/$fileName.tar.xz * $extraArgs
mkdir -p $out/nix-support
echo $system > $out/nix-support/system
···
rm env-vars
+
tar --sort=name --mtime='@1' --owner=0 --group=0 --numeric-owner -cvJf $out/tarball/$fileName.tar.xz * $extraArgs
mkdir -p $out/nix-support
echo $system > $out/nix-support/system