Make it easier to append to the default sudo configuration

modules/security/sudo.nix

@@ -37,7 +37,20 @@ in
     security.sudo.configFile = mkOption {
       # Note: if syntax errors are detected in this file, the NixOS
       # configuration will fail to build.
-      default =
+      description =
+        ''
+          This string contains the contents of the
+          <filename>sudoers</filename> file.
+        '';
+    };
+  };
+
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+
+    security.sudo.configFile =
       ''
         # Don't edit this file. Set the NixOS option ‘security.sudo.configFile’ instead.
 
@@ -56,18 +69,6 @@ in
         # Users in the "wheel" group can do anything.
         %wheel      ALL=(ALL) ${if cfg.wheelNeedsPassword then "" else "NOPASSWD: ALL, "}SETENV: ALL
       '';
-      description =
-        ''
-          This string contains the contents of the
-          <filename>sudoers</filename> file.
-        '';
-    };
-  };
-
-
-  ###### implementation
-
-  config = mkIf cfg.enable {
 
     security.setuidPrograms = [ "sudo" ];