iso_gnome: Fix evaluation

`qt.enable` option requires `qt.style` to be set.
Previously, this was set in GNOME module but it has been removed
in 622745942bc7b7cc056bfbb0bc6004dd823fa4f5

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

···
     enable = true;
   };
 
-  # Theme calamares with GNOME theme
-  qt = {
-    enable = true;
-    platformTheme = "gnome";
-  };
-
   # Fix scaling for calamares on wayland
   environment.variables = {
     QT_QPA_PLATFORM = "$([[ $XDG_SESSION_TYPE = \"wayland\" ]] && echo \"wayland\")";