nixos/tools: add copySystemConfiguration to configuration file template

nixos/modules/installer/tools/tools.nix

@@ -206,6 +206,11 @@ in
         # Or disable the firewall altogether.
         # networking.firewall.enable = false;
 
+        # Copy the NixOS configuration file and link it from the resulting system
+        # (/run/current-system/configuration.nix). This is useful in case you
+        # accidentally delete configuration.nix.
+        # system.copySystemConfiguration = true;
+
         # This value determines the NixOS release from which the default
         # settings for stateful data, like file locations and database versions
         # on your system were taken. It‘s perfectly fine and recommended to leave