Use test in build matrix too

Author: joecummings

.github/workflows/build_vllm.yaml

@@ -39,7 +39,7 @@ jobs:
               "build_name": "manywheel-cuda12_8",
               "validation_runner": "linux.12xlarge.memory",
               "installation": "pip3 install torch torchvision torchaudio --index-url https://download.pytorch.org/whl/cu128",
-              "channel": "nightly",
+              "channel": "test",
               "upload_to_base_bucket": "no",
               "stable_version": "2.9.0",
               "use_split_build": false