fix typo in getenv call (vllm-project#1972)

Author: dskhudia

setup.py

@@ -138,7 +138,7 @@ def get_torch_arch_list() -> Set[str]:
 
 # Use NVCC threads to parallelize the build.
 if nvcc_cuda_version >= Version("11.2"):
-    nvcc_threads = int(os.getenv("NVCC_THREADS"), 8)
+    nvcc_threads = int(os.getenv("NVCC_THREADS", 8))
     num_threads = min(os.cpu_count(), nvcc_threads)
     NVCC_FLAGS += ["--threads", str(num_threads)]