changefeedccl: deflake TestChangefeedRetryableSinkError

This channel wasn't getting closed if the stopper rejected the async
task, which casued `(*changeAggregator).close()` to hang.

Release note: None

pkg/ccl/changefeedccl/changefeed_processors.go

@@ -166,8 +166,9 @@ func (ca *changeAggregator) Start(ctx context.Context) context.Context {
 	// Give errCh enough buffer both possible errors from supporting goroutines,
 	// but only the first one is ever used.
 	ca.errCh = make(chan error, 2)
-	ca.pollerDoneCh = make(chan struct{})
+
 	if err := ca.flowCtx.Stopper().RunAsyncTask(ctx, "changefeed-poller", func(ctx context.Context) {
+		ca.pollerDoneCh = make(chan struct{})
 		defer close(ca.pollerDoneCh)
 		var err error
 		if storage.RangefeedEnabled.Get(&ca.flowCtx.Settings.SV) {