nixos/iso-image: Re-enable graphics mode

Which ***anyway*** was not disabled correctly. Following changes will
actually disable it.

What this did was disable the "themed" menu driver, but still continued
relying on the gfxterm infra, which in itself is why things were ugly
and weird.

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -713,7 +713,7 @@ in
     };
 
     isoImage.graphicalGrub = mkOption {
-      default = false;
+      default = true;
       type = types.bool;
       example = true;
       description = lib.mdDoc ''