Added USE_NCCL_LIB_PATH option to enable user to set NCCL_LIBRARY during build  (#6310)
Added USE_NCCL_LIB_PATH option to enable user to set NCCL_LIBRARY during build  (#6310)

Description: To enable user to set NCCL_LIBRARY during build
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost