nix-darwin/modules/aspects/activation.nix

{...}: {
  flake.modules.darwin.activation = {
    config,
    lib,
    pkgs,
    ...
  }: {
    system.activationScripts.extraActivation.text = ''
      sudo -u ${config.repo.user.name} /System/Library/PrivateFrameworks/SystemAdministration.framework/Resources/activateSettings -u
    '';

    system.activationScripts.applications.text = let
      env = pkgs.buildEnv {
        name = "system-applications";
        paths = config.environment.systemPackages;
        pathsToLink = ["/Applications"];
      };
    in
      lib.mkForce ''
        echo "setting up /Applications..." >&2
        rm -rf /Applications/Nix\ Apps
        mkdir -p /Applications/Nix\ Apps
        find ${env}/Applications -maxdepth 1 -type l -exec readlink '{}' + |
        while read -r src; do
          app_name=$(basename "$src")
          echo "copying $src" >&2
          ${pkgs.mkalias}/bin/mkalias "$src" "/Applications/Nix Apps/$app_name"
        done
      '';
  };
}