nixos/iso: enable partition-manager (for kpmcore)

nixos/modules/installer/cd-dvd/installation-cd-graphical-calamares.nix

···
 {
   imports = [ ./installation-cd-graphical-base.nix ];
 
+  # required for kpmcore to work correctly
+  programs.partition-manager.enable = true;
+
   environment.systemPackages = with pkgs; [
     # Calamares for graphical installation
-    libsForQt5.kpmcore
     calamares-nixos
     calamares-nixos-autostart
     calamares-nixos-extensions