[CI]Upgrade niglty multi-node-tests max-parallel to 2 (#7035)

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

1. Increase nightly multi-node test max-parallel from 1 to 2, and fix
resource conflicts that arise when tests run concurrently.
2. Fix parse-trigger job: Add an if condition so it only runs on
schedule, workflow_dispatch, or PRs labeled nightly-test
3. Adjust nightly schedule: Shift trigger time from 24:00 to 23:45
(UTC+8)

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

### How was this patch tested?

- vLLM version: v0.16.0
- vLLM main:
4034c3d32e

---------

Signed-off-by: hfadzxy <starmoon_zhang@163.com>

.github/workflows/schedule_nightly_test_a3.yaml

@@ -22,8 +22,8 @@ name: Nightly-A3
 
 on:
   schedule:
-      # Run test at 24:00 Beijing time (UTC+8)
-      - cron: "0 16 * * *"
+      # Run test at 23:45 Beijing time (UTC+8)
+      - cron: "45 15 * * *"
   workflow_dispatch:
   pull_request:
     branches:
@@ -50,6 +50,10 @@ jobs:
   parse-trigger:
     name: Parse trigger and determine test scope
     runs-on: linux-aarch64-a2b3-0
+    if: >-
+      github.event_name == 'schedule' ||
+      github.event_name == 'workflow_dispatch' ||
+      contains(github.event.pull_request.labels.*.name, 'nightly-test')
     outputs:
       should_run: ${{ steps.parse.outputs.should_run }}
       test_filter: ${{ steps.parse.outputs.test_filter }}
@@ -127,7 +131,7 @@ jobs:
     if: always() && needs.parse-trigger.outputs.should_run == 'true'
     strategy:
       fail-fast: false
-      max-parallel: 1
+      max-parallel: 2
       matrix:
         test_config:
           - name: multi-node-deepseek-pd