fix eval_harness download cache-dir

[pihao7]

This is to fix a issue when run evaluation stage on DGX cloud,

python3 /opt/NeMo-Megatron-Launcher/launcher_scripts/main.py \
             data_dir=/mount_workspace/data  stages=[evaluation] cluster_type=bcp \
             ...

the above script would generate bcprun command like this

bcprun --nnodes 1 --npernode 1 -no_redirect  --env 'TRANSFORMERS_OFFLINE=1' --env 'TORCH_NCCL_AVOID_RECORD_STREAMS=1' --env 'NCCL_NVLS_ENABLE=0' --cmd "
  python3 /opt/NeMo-Megatron-Launcher/launcher_scripts/nemo_launcher/collections/eval_harness/download.py \
  --tasks=all_tasks \
  --cache-dir=/mount_workspace/data/eval_harness_data"

but the eval datasets are not download to the cache-dir, because the download.py instead expected underscore version of cache_dir.

[dimapihtar]

LGTM. Thank you!