nixos/doas: default rule should be first

In /etc/doas.conf, the last-matched rule will override all
previously-matched rules. Thus, make the default rule show up first (but
still allow some wiggle room for a user to `mkBefore` it), before any
user-defined rules.

nixos/modules/security/doas.nix

@@ -223,7 +223,7 @@ in
 
   config = mkIf cfg.enable {
 
-    security.doas.extraRules = [
+    security.doas.extraRules = mkOrder 600 [
       {
         groups = [ "wheel" ];
         noPass = !cfg.wheelNeedsPassword;