[CI] Revert speedup image building and CI Installation related PRs (#6891)

### What this PR does / why we need it?

Revert speedup image building and CI Installation related PRs

git revert 8835236181
git revert 64fba51275
git revert 263c2f8e8d
git revert 84b00695f8


### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

---------

Signed-off-by: wjunLu <wjunlu217@gmail.com>

.github/workflows/schedule_image_build_and_push.yaml

@@ -12,15 +12,12 @@
 name: Image Build and Push
 on:
   schedule:
-    # UTC+8: 8am, 10am, 12pm, 14pm, 16pm, 18pm, 22pm
-    - cron: '0 0,2,4,6,8,10,14 * * *'
+    # UTC+8: 8am, 12pm, 16pm, 22pm
+    - cron: '0 0,4,8,14 * * *'
   push:
-    branches:
-      # Build release branch images proactively so cache is warm when the tag is pushed
-      - 'releases/*'
     tags:
       - 'v*'
-  pull_request:
+  pull_request: 
     branches:
       - 'main'
     types: [ labeled, synchronize ]