[BUG] System Prompt option now missing?

## Describe the bug
I was using an older version of LightEval packaged with OpenR1 repo, it had a `--system-prompt "xxx"` arg for lighteval.

That seems to be gone. I'm told that it is now a model arg, though I dont see that in the docs for vllm, but also if any comma is in the system prompt, it will break everything.

## To Reproduce
use

```
lighteval vllm "$MODEL_ARGS" "$benchmark_task" \
--system-prompt "$system_prompt" \
--use-chat-template \
--output-dir "$output_dir" 2>&1 | tee -a "$log_file"
```