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/castore/src/import.rs

@@ -122,7 +122,7 @@ async fn process_entry<'a>(
         return Ok(Node::File(FileNode {
             name: entry.file_name().as_bytes().to_vec().into(),
             digest: digest.into(),
-            size: metadata.len() as u32,
+            size: metadata.len(),
             // If it's executable by the user, it'll become executable.
             // This matches nix's dump() function behaviour.
             executable: metadata.permissions().mode() & 64 != 0,