install agenix to system and devShell

flake.nix

@@ -312,6 +312,7 @@
       default = pkgs.mkShell {
         buildInputs = with pkgs; [
           nix-update
+          inputs.agenix.packages.${system}.default
         ];
       };
     });

system/default.nix

@@ -18,6 +18,7 @@ in
     # modules
     inputs.nix-minecraft.nixosModules.minecraft-servers
     inputs.impermanence.nixosModules.impermanence
+    inputs.agenix.nixosModules.default
     ./stylix.nix # imports inputs.stylix
   ];
   config = {};