[CI] Fix nighly command (vllm-project#11358)

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

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

### How was this patch tested?

- 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

@@ -154,8 +154,8 @@ jobs:
           echo "test_cases=$TEST_CASES" >> "$GITHUB_OUTPUT"
           echo "branch=$BRANCH" >> "$GITHUB_OUTPUT"
 
-          HEAD_REPO=$(gh api "$PR_URL" --jq '.head.repo.full_name')
           PR_SHA=$(gh api "$PR_URL" --jq '.head.sha')
+          PR_NUMBER=$(gh api "$PR_URL" --jq '.number')
           echo "vllm_ascend_ref=$PR_SHA" >> "$GITHUB_OUTPUT"
 
           if [ "$TEST_CASES" = "all" ]; then
@@ -164,19 +164,22 @@ jobs:
             exit 0
           fi
 
-          # Fetch the matching test matrix from the exact PR commit (not branch HEAD,
-          # to match vllm_ascend_ref). Nightly reads nightly_config.yaml; weekly reads
-          # weekly_config.yaml. resolve_nightly_tests.py picks the non-empty one.
+          # Fetch the matching test matrix from the PR head ref via pull/<num>/head,
+          # which is accessible through the base repo without fork access permission.
+          # Nightly reads nightly_config.yaml; weekly reads weekly_config.yaml.
+          # resolve_nightly_tests.py picks the non-empty one.
+          REPO='${{ github.event.client_payload.github.payload.repository.full_name }}'
           case "$COMMAND" in
             nightly)
-              NIGHTLY_MATRIX=$(gh api "repos/$HEAD_REPO/contents/.github/workflows/configs/nightly_config.yaml?ref=$PR_SHA" --jq '.content' 2>/dev/null || echo "")
+              NIGHTLY_MATRIX=$(gh api "repos/$REPO/contents/.github/workflows/configs/nightly_config.yaml?ref=pull/$PR_NUMBER/head" --jq '.content' 2>/dev/null || echo "")
               ;;
             weekly)
-              WEEKLY_MATRIX=$(gh api "repos/$HEAD_REPO/contents/.github/workflows/configs/weekly_config.yaml?ref=$PR_SHA" --jq '.content' 2>/dev/null || echo "")
+              WEEKLY_MATRIX=$(gh api "repos/$REPO/contents/.github/workflows/configs/weekly_config.yaml?ref=pull/$PR_NUMBER/head" --jq '.content' 2>/dev/null || echo "")
               ;;
           esac
           export NIGHTLY_MATRIX
           export WEEKLY_MATRIX
+          export TEST_CASES
 
           python3 .github/workflows/scripts/resolve_nightly_tests.py --mode=dispatch