MM-53747: Do not start if job is in-progress as well. (#24115)

We missed this out last time. It's possible in an HA
scenario for a second pod to start later while the other
job is in-progress. In that case, it would schedule
two jobs.

https://mattermost.atlassian.net/browse/MM-53747

```release-note
NONE
```

server/channels/store/retrylayer/retrylayer.go

@@ -5635,6 +5635,27 @@ func (s *RetryLayerJobStore) Save(job *model.Job) (*model.Job, error) {
 
 }
 
+func (s *RetryLayerJobStore) SaveOnce(job *model.Job) (*model.Job, error) {
+
+	tries := 0
+	for {
+		result, err := s.JobStore.SaveOnce(job)
+		if err == nil {
+			return result, nil
+		}
+		if !isRepeatableError(err) {
+			return result, err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return result, err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerJobStore) UpdateOptimistically(job *model.Job, currentStatus string) (bool, error) {
 
 	tries := 0