nixos gtk.iconCache.enable: default from xserver.enable

It's a quick approximation to unblock unstable channels after #48116.
This commit isn't ideal, as I suspect most wayland users won't have
xserver.enable, so they will lose the icon cache in case they had gtk
in system path (otherwise they didn't get cache anyway).

I considered using environment.noXlibs, but the nixos tests installing
headless systems do *not* get that option, so we would still be pulling
gtk in many cases where it's clearly not desired.  We need to design
this more carefully.

nixos/modules/config/gtk/gtk-icon-cache.nix

@@ -5,7 +5,7 @@ with lib;
   options = {
     gtk.iconCache.enable = mkOption {
       type = types.bool;
-      default = true;
+      default = config.services.xserver.enable;
       description = ''
         Whether to build icon theme caches for GTK+ applications.
       '';