ci: run update workflow every 1st of month

Signed-off-by: Qiming Chu <cchuqiming@gmail.com>

Author: Emin017

.github/workflows/update.yml

@@ -1,8 +1,8 @@
 name: Update flake lock
 on:
   schedule:
-    # Trigger every Monday and Thursday at 8:00 UTC
-    - cron: '0 8 * * 1'
+    # Trigger every month on the 1st at 08:00 UTC
+    - cron: '0 8 1 * *'
   workflow_dispatch: # Allow manual triggering
 jobs:
   update-flake-lock: