Create helper function to create NixNG images

Move NixNG images to separate directory

utils.nix

@@ -0,0 +1,19 @@
+{ pkgs, nixpkgs, nixng, globals, ... }: {
+  mkNixNGImage = name: file:
+    let
+      stream = (import file {
+        inherit nixpkgs nixng globals;
+        inherit (nixng) nglib;
+      }).config.system.build.ociImage.stream;
+    in
+    pkgs.stdenv.mkDerivation {
+      name = "${name}.tar";
+      src = stream;
+      dontUnpack = true;
+      buildPhase = ''
+        $src > $out
+      '';
+    };
+
+  nixSnapshotterRef = imagePath: "nix:0${imagePath}";
+}