feat(worker): add service log filepath (#1640)

worker/install/systemd/libretime-celery.service

@@ -2,6 +2,7 @@
 Description=LibreTime Worker Service
 
 [Service]
+Environment=LIBRETIME_LOG_FILEPATH=/var/log/libretime/worker.log
 Environment=LIBRETIME_CONFIG_FILEPATH=/etc/airtime/airtime.conf
 WorkingDirectory=/var/lib/libretime/worker
 
@@ -10,7 +11,8 @@ ExecStart=/usr/local/bin/celery worker \
     --config=libretime_worker.config \
     --time-limit=1800 \
     --concurrency=1 \
-    --loglevel=INFO
+    --loglevel=INFO \
+    --logfile=$LIBRETIME_LOG_FILEPATH
 User=libretime-worker
 Group=libretime-worker
 Restart=always