enable NetworkManager OpenVPN

modules/capabilities/networking.nix

@@ -11,7 +11,10 @@ in
     networking.networkmanager.enable = true;
   };
 
-  flake.modules.nixos.openvpn = {
+  flake.modules.nixos.openvpn =
+    { pkgs, ... }:
+    {
+      networking.networkmanager.plugins = [ pkgs.networkmanager-openvpn ];
       programs.openvpn3.enable = true;
       services.resolved.enable = true;
     };