sdk/rust: Fix pwrite to flushing buffer before returning

Let's call flush() after write_all() to be safe. It's unclear to me if
this explains a hostfs test failure I saw, but Tokio's documentation
suggests that a write_all() returns when the write is handed over to the
thread pool, but makes no guarantees that the write actually is done,
meaning a subsequent read might return state data.

Author: penberg

sdk/rust/src/filesystem/hostfs.rs

@@ -131,6 +131,7 @@ impl FileSystem for HostFS {
 
         file.seek(std::io::SeekFrom::Start(offset)).await?;
         file.write_all(data).await?;
+        file.flush().await?;
         Ok(())
     }