[CI] Avoid lint and ut for PR push (#5762)

1. Don't run lint and ut again once the PR is merged to save CI resource
2. Update codecov every 4 hour
3. rename `model_downloader` to suitable name
4. update schedule job to better time.

- vLLM version: v0.13.0
- vLLM main:
2f4e6548ef

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

.github/workflows/schedule_image_build_and_push.yaml

@@ -12,7 +12,8 @@
 name: Image Build and Push
 on:
   schedule:
-    - cron: '0 */4 * * *'
+    # UTC+8: 8am, 12pm, 16pm, 22pm
+    - cron: '0 0,4,8,14 * * *'
   push:
     tags:
       - 'v*'