nixos/sudo: Guard against security.sudo.package = pkgs.sudo-rs;

This is not unlikely to happen, given the enthusiasm shown by some users,
but we are not there yet, and this will save them from breaking their system.

nixos/modules/security/sudo.nix

@@ -192,6 +192,10 @@ in
   ###### implementation
 
   config = mkIf cfg.enable {
+    assertions = [
+      { assertion = cfg.package.pname != "sudo-rs";
+        message = "The NixOS `sudo` module does not work with `sudo-rs` yet."; }
+    ];
 
     # We `mkOrder 600` so that the default rule shows up first, but there is
     # still enough room for a user to `mkBefore` it.