Do not fetch NCCL when system NCCL is used (pytorch#149607)

We are compiling PyTorch in a sandbox without networking. Unconditionally fetching breaks the build and is not needed when a system NCCL is used.
Pull Request resolved: pytorch#149607
Approved by: https://github.com/malfet

Author: danieldk

tools/build_pytorch_libs.py

@@ -119,7 +119,8 @@ def build_pytorch(
     cmake: CMake,
 ) -> None:
     my_env = _create_build_env()
-    checkout_nccl()
+    if os.getenv("USE_SYSTEM_NCCL", "0") == "0":
+        checkout_nccl()
     build_test = not check_negative_env_flag("BUILD_TEST")
     cmake.generate(
         version, cmake_python_library, build_python, build_test, my_env, rerun_cmake