Fix regex `get_data_files` formatting for base paths
With this pr https://github.com/huggingface/datasets/pull/6309, it is formatting the entire base path into regex, which results in the undesired formatting error `doesn't match the pattern` because of the line in `glob_pattern_to_regex`: `.replace("//", "/")`:
- Input: `hf://datasets/...`
- Output: `hf:/datasets/...`

This fix will only convert the `split_pattern` to regex and keep the `base_path` unchanged.

cc @albertvillanova hopefully this still works with your implementation