flake: add a new follow override for the mailserver

flake.nix

@@ -19,6 +19,7 @@
     # note that in theory doing this might break things, but it seems fairly unlikely
     nixos-mailserver.inputs = {
       nixpkgs.follows = "nixpkgs-unstable";
+      nixpkgs_24-11.follows = "nixpkgs";
       flake-compat.follows = "/deploy-rs/flake-compat";
     };
   };