Fix polling writing to log file (librenms#17983)

used by dispatcher "watchdog"

Author: murrant

app/Jobs/PollDevice.php

@@ -90,7 +90,7 @@ public function handle(): void
             $measurement->getDuration()));
 
         // add log file line, this is used by the simple python dispatcher watchdog
-        Log::channel('single')->alert(sprintf('INFO: device:poll %s (%s) polled in %0.3fs',
+        Log::channel('log_file')->alert(sprintf('INFO: device:poll %s (%s) polled in %0.3fs',
             $this->device->hostname,
             $this->device->device_id,
             $measurement->getDuration()));