Fix cell truncation
We were using the same `list` for all the cells. So if a cell has truncated columns then all the cells end up with the same value for `truncated_cells`

Close https://github.com/huggingface/datasets-server/issues/2586
Fix cell truncation: We were using the same `list` for all the cells. So if a cell has truncated columns then all the cells end up with the same value for `truncated_cells`

Close https://github.com/huggingface/datasets-server/issues/2586