[CI] Uncomment condition for nightly build in YAML (#2057)

Signed-off-by: Gao Han <hgaoaf@connect.ust.hk>

.buildkite/test-nightly.yml

@@ -73,7 +73,7 @@ steps:
   - label: ":full_moon: Diffusion Model Test with H100"
     timeout_in_minutes: 60
     depends_on: upload-nightly-pipeline
-    # if: build.env("NIGHTLY") == "1"
+    if: build.env("NIGHTLY") == "1"
     commands:
       - export VLLM_WORKER_MULTIPROC_METHOD=spawn
       - pytest -s -v tests/e2e/online_serving/test_*_expansion.py -m "advanced_model and diffusion and H100" --run-level "advanced_model"