Use old Morph and new Vagrant consistently

shell.nix

@@ -2,11 +2,13 @@ let
   nixpkgs-pin = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
   nixpkgs-src = builtins.fetchTarball nixpkgs-pin;
   nixpkgs = import nixpkgs-src { };
+  stable2105 = import (builtins.fetchTarball https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz) { };
 in
 { pkgs ? nixpkgs }:
 pkgs.mkShell {
   NIX_PATH = "nixpkgs=${nixpkgs.path}";
   buildInputs = [
     pkgs.morph
+    stable2105.vagrant
   ];
 }