nixos/amazon-image: (temporarily) use fixed disk size again

As a temporary workaround for #120473 while the image builder is patched
to correctly look up disk sizes, partially revert
f3aa040bcb for EC2 disk images only.

We retain the type allowing "auto" but set the default back to the
previous value.

nixos/maintainers/scripts/ec2/amazon-image.nix

@@ -41,7 +41,8 @@ in {
 
     sizeMB = mkOption {
       type = with types; either (enum [ "auto" ]) int;
-      default = "auto";
+      # TODO(lukegb): this should be "auto"; see #120473
+      default = if config.ec2.hvm then 2048 else 8192;
       example = 8192;
       description = "The size in MB of the image";
     };