fix not reload job config

nanobot/cron/service.py

@@ -84,6 +84,7 @@ class CronService:
             if mtime != self._last_mtime:
                 logger.info("Cron: jobs.json modified externally, reloading")
                 self._store = None
+                self._last_mtime = mtime
         if self._store:
             return self._store
 
@@ -190,8 +191,7 @@ class CronService:
         }
 
         self.store_path.write_text(json.dumps(data, indent=2, ensure_ascii=False), encoding="utf-8")
-        self._last_mtime = self.store_path.stat().st_mtime
-    
+
     async def start(self) -> None:
         """Start the cron service."""
         self._running = True