Ensure CheckpointBuilder can be aborted at end of epoch (#20442)

crates/sui-core/src/checkpoints/mod.rs

@@ -985,6 +985,9 @@ impl CheckpointBuilder {
                 self.metrics.checkpoint_errors.inc();
                 return;
             }
+            // ensure that the task can be cancelled at end of epoch, even if no other await yields
+            // execution.
+            tokio::task::yield_now().await;
         }
         debug!(
             "Waiting for more checkpoints from consensus after processing {last_height:?}; {} pending checkpoints left unprocessed until next interval",