[ci] mark blackwell test optional for now (vllm-project#21878)

Author: simon-mo

.buildkite/test-pipeline.yaml

@@ -646,7 +646,7 @@ steps:
 - label: Blackwell Test
   working_dir: "/vllm-workspace/"
   gpu: b200
-  # optional: true
+  optional: true
   source_file_dependencies:
   - csrc/
   - vllm/