fix(tvix/castore): B3Digest::{to_vec -> as_slice}

Not a single call site actually makes use of the Vec.

Change-Id: I6cf31073c9f443d1702a21937a0c3938c2c643b8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9988
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/castore/src/proto/grpc_blobservice_wrapper.rs

@@ -167,8 +167,7 @@ impl super::blob_service_server::BlobService for GRPCBlobServiceWrapper {
                 warn!("error closing stream: {}", e);
                 Status::internal("error closing stream")
             })
-            .await?
-            .to_vec();
+            .await?;
 
         Ok(Response::new(super::PutBlobResponse {
             digest: digest.into(),