nix/nixos-module/defaults: use non-nft iptables-legacy for migration

nix/nixos-module/defaults.nix

@@ -54,6 +54,11 @@
     };
   };
 
+  nixpkgs.config.packageOverrides = pkgs: {
+    # TODO: remove when host is rebooted with a nixos-21.11 kernel
+    iptables = pkgs.iptables-legacy;
+  };
+
   documentation = {
     enable = false;
     nixos.enable = false;