[CI] fix image build tag (#5703)

ref doesn't work with workflow_dispatch, let's change it to raw way

This PR also merge the pr_create job into one runner to save resource.
- 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 +12,7 @@
 name: Image Build and Push
 on:
   schedule:
-    - cron: '0 */2 * * *'
+    - cron: '0 */4 * * *'
   push:
     tags:
       - 'v*'