refactor(tvix/castore): use structured logging for directory digest

See https://cl.tvl.fyi/c/depot/+/13155/comment/7bb9b399_e29d929a/

Change-Id: I641e5c630fb7afdc41fb7e4dc79115697f702b27
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13159
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>

tvix/castore/src/directoryservice/traverse.rs

@@ -24,7 +24,7 @@ where
                 // fetch the linked node from the directory_service.
                 let directory = directory_service.get(&digest).await?.ok_or_else(|| {
                     // If we didn't get the directory node that's linked, that's a store inconsistency, bail out!
-                    warn!("directory {} does not exist", digest);
+                    warn!(directory.digest = %digest, "directory does not exist");
 
                     Error::StorageError(format!("directory {} does not exist", digest))
                 })?;