[CI] correct event action path in pr_nightly_command workflow (vllm-project#11339)

### What this PR does / why we need it?
correct event action path in pr_nightly_command workflow

- vLLM version: v0.23.0
- vLLM main:
vllm-project/vllm@a30addc

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

Author: zhangxinyuehfad

.github/workflows/pr_nightly_command.yml

@@ -113,7 +113,7 @@ jobs:
           PR_URL: ${{ github.event.client_payload.github.payload.issue.pull_request.url }}
         run: |
           # Derive command type from repository_dispatch action (e.g. nightly-command -> nightly)
-          COMMAND='${{ github.event.repository_dispatch.action }}'
+          COMMAND='${{ github.event.action }}'
           COMMAND="${COMMAND%-command}"
           echo "command=$COMMAND" >> "$GITHUB_OUTPUT"