nixos/virtualbox-image: set the root fsType to reenable root FS resizing

This otherwise does not eval `:tested` any more, which means no nixos
channel updates.

Regression comes from 0eb6d0735f599fa84e99b727c93681bcae96c845 (#57751)
which added an assertion stopping the use of `autoResize` when the
filesystem cannot be resized automatically.

nixos/modules/virtualisation/virtualbox-image.nix

···
     fileSystems."/" = {
       device = "/dev/disk/by-label/nixos";
       autoResize = true;
+      fsType = "ext4";
     };
 
     boot.growPartition = true;