AppImage and polkit (Outline VPN client)

I’m trying to install Outline VPN Client on NixOs. For this I have downloaded Outline-Client.AppImage from here (for Linux) and have written the following derivation:

with import <nixpkgs> {};

let app = appimageTools.wrapType2 {
      name = "outline-client";
      src = ./Outline-Client.AppImage;
  stdenv.mkDerivation {
    name =  "outline-to-scope";
    buildInputs = [ app ];

(saved in file outline-client.nix and have Outline-Client.Appimage in the same folder). When I run

$ nix-shell outline-client.nix
$ outline-client

the outline client starts and I can add a key, but if I try to connect, it can’t with the following error in console:

command is running as root but failed:  Error: Unable to find pkexec or kdesudo. at e (/nix/store/f1..bx5-outline-client-extracted/resources/app.asar/build/electron/electron/index.js:2:408556) at /nix/store/f1..bx5-outline-client-extracted/resource

I have polkit enabled and pkexec command is /run/wrappers/bin/pkexec.
How can I install the Outline client properly?