cdc: avoid deadlock on error in pubsub sink

cockroachdb#88130
introduced a deadlock when an attempt to create a
topic fails -- the goroutine tries to acquire a lock
in order to record the error, but it already has it
in order to write to the map. This PR releases the lock
while creating the topic, which should also help with
performance a bit on startup.

Release note (bug fix): Fixed a bug preventing pubsub changefeeds from retrying.

pkg/ccl/changefeedccl/sink_pubsub.go

@@ -355,9 +355,12 @@ func (p *gcpPubsubClient) getTopicClient(name string) (*pubsub.Topic, error) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 	if topic, ok := p.topics[name]; ok {
+		p.mu.Unlock()
 		return topic, nil
 	}
+	p.mu.Unlock() // openTopic may need the lock to record an error
 	topic, err := p.openTopic(name)
+	p.mu.Lock()
 	if err != nil {
 		return nil, err
 	}