[Nightly] Nightly pre-build image (#7388)

### What this PR does / why we need it?
This pull request refactor nightly image build and simplify the logic of
multi workflows.
1. Nightly image build become the prerequisite when the test are
triggered by `schedule` or `workflow_dispatch`
2. Simplify the pull request select case logic
3. Next step: Implement replaceable nightly tests. Specifically, if
nightly tests are manually triggered, they can accept any optional
docker image to meet the needs of different commits(Which means the
image is customizable).
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.17.0
- vLLM main:
4034c3d32e

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

.github/workflows/_e2e_nightly_multi_node.yaml

@@ -45,12 +45,9 @@ on:
         default: main
         type: string
         description: used for pr level tests
-      is_pr_test:
+      should_run:
         required: true
         type: boolean
-      is_run:
-        required: true  
-        type: boolean
     secrets:
       KUBECONFIG_B64:
         required: true
@@ -74,7 +71,7 @@ jobs:
     name: ${{ inputs.config_file_path }}
     # This is the runner with no NPU for k8s controller
     runs-on: ${{ inputs.runner }}
-    if: ${{ inputs.is_run }}
+    if: ${{ inputs.should_run }}
     container:
       image: swr.cn-southwest-2.myhuaweicloud.com/base_image/ascend-ci/vllm-ascend:nightly-cpu
       env:
@@ -84,7 +81,7 @@ jobs:
         - name: Decode kubeconfig from secrets
           run: |
             # Decode and save kubeconfig
-            if [ "${{ inputs.is_pr_test }}" = "true" ]; then
+            if [ "${{ github.event_name }}" = "pull_request" ]; then
               echo "PR test mode"
               if [ "${{ inputs.soc_version }}" = "a3" ]; then
                 echo "Using A3 cached kubeconfig"
@@ -161,7 +158,7 @@ jobs:
             image="${{ inputs.image }}"
             config_file_path="${{ inputs.config_file_path }}"
             fail_tag=FAIL_TAG_"${{ inputs.config_file_path }}"
-            is_pr_test="${{ inputs.is_pr_test }}"
+            is_pr_test="${{ github.event_name == 'pull_request' }}"
             vllm_version="${{ inputs.vllm_version }}"
             vllm_ascend_ref="${{ inputs.vllm_ascend_ref }}"
             vllm_ascend_remote_url="${{ inputs.vllm_ascend_remote_url }}"