[CI/Build][Bugfix] VLLM_USE_PRECOMPILED should skip compilation (#4534)

Signed-off-by: Travis Johnson <tsjohnso@us.ibm.com>

setup.py

@@ -378,6 +378,7 @@ package_data = {
     "vllm": ["py.typed", "model_executor/layers/fused_moe/configs/*.json"]
 }
 if os.environ.get("VLLM_USE_PRECOMPILED"):
+    ext_modules = []
     package_data["vllm"].append("*.so")
 
 setup(