[CI] Add env variable check for nightly CI (vllm-project#1281)

Signed-off-by: Alicia <115451386+congw729@users.noreply.github.com>

Author: congw729

.buildkite/test-nightly.yaml

@@ -12,6 +12,7 @@ steps:
   - label: "Omni Model Test with H100"
     timeout_in_minutes: 180
     depends_on: image-build
+    if: build.env("NIGHTLY") == "1"
     commands:
       - export VLLM_WORKER_MULTIPROC_METHOD=spawn
       - pytest -s -v tests/e2e/online_serving/test_qwen3_omni_expansion.py