Set a default host platform

A nixos config only needs a few things set in order to build correctly,
`nixpkgs.hostPlatform` being one of them. In the case of jetson devices,
we can at least provide a default value for this that gets people up and
running with one less thing to set.

modules/default.nix

@@ -91,6 +91,8 @@ in
       message = "Docker version < 25 does not support CDI";
     }];
 
+    nixpkgs.hostPlatform = lib.mkDefault "aarch64-linux";
+
     nixpkgs.overlays = [
       (import ../overlay.nix)
     ];