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.