feat(tvix/castore): bump [Directory,File]Node size to u64

Having more than 4GiB files is quite possible (think about the NixOS
graphical installer, and an uncompressed iso of it).

No wire format changes.

Change-Id: Ia78a07e4c554e91b93c5b9f8533266e4bd7f22b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9950
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/src/nar/renderer.rs

@@ -136,7 +136,7 @@ async fn walk_node(
             nar_node
                 .file(
                     proto_file_node.executable,
-                    proto_file_node.size.into(),
+                    proto_file_node.size,
                     &mut blob_reader.compat(),
                 )
                 .await