Add nix-serve-ng to outputs; add .envrc for flake direnv

flake.nix

@@ -15,7 +15,7 @@
     };
   };
 
-  outputs = { self, nixpkgs, agenix, ... }@attrs:
+  outputs = { self, nixpkgs, agenix, nix-serve-ng, ... }@attrs:
     let
       pkgs = import nixpkgs { inherit system; };
       shell = import ./shell.nix { inherit agenix pkgs; };