Use old morph for now even in newer environment

shell.nix

@@ -6,7 +6,7 @@ in
 pkgs.mkShell {
   NIX_PATH = "nixpkgs=${pkgs.path}";
   buildInputs = [
-    pkgs.morph
+    nixpkgs.morph
     stable2105.vagrant
     pkgs.jp
   ];