nix-darwin-activation.nix

system.activationScripts.applications.text = let
  env = pkgs.buildEnv {
    name = "system-applications";
    paths = config.environment.systemPackages;
    pathsToLink = "/Applications";
  };
in
  pkgs.lib.mkForce ''
  # Set up applications.
  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
      '';

for the latest nix version the function pkgs.buildEnv expects pathsToLink to be a list (like ["/A"]).

pathsToLink = ["/Applications"];