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/timerlayer/timerlayer.go

@@ -4534,6 +4534,22 @@ func (s *TimerLayerJobStore) Save(job *model.Job) (*model.Job, error) {
 	return result, err
 }
 
+func (s *TimerLayerJobStore) SaveOnce(job *model.Job) (*model.Job, error) {
+	start := time.Now()
+
+	result, err := s.JobStore.SaveOnce(job)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("JobStore.SaveOnce", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerJobStore) UpdateOptimistically(job *model.Job, currentStatus string) (bool, error) {
 	start := time.Now()