Fix counting in Slack report for some jobs (#22913)

fix
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

utils/notification_service.py

@@ -655,7 +655,7 @@ class Message:
                         job_result,
                         failures,
                         device,
-                        text=f"Number of failures: {sum(job_result['failed'].values())}",
+                        text=f'Number of failures: {job_result["failed"][device]}',
                     )
 
                     print("Sending the following reply")