Forcefully shutdown the runtime before existing the main task (microsoft#3378)

Author: chkeita

src/agent/onefuzz-task/src/main.rs

@@ -40,6 +40,7 @@ fn main() -> Result<()> {
     let rt = tokio::runtime::Runtime::new()?;
     let result = rt.block_on(run(matches));
     atexit::execute();
+    rt.shutdown_background();
     result
 }