set updated_at field to the same value for all steps of a dataset
After the migrations:
- `_20240221103200_cache_merge_config_split_names`
- `_20240221160700_cache_merge_split_first_rows`

the updated_at field is not accurate anymore, leading to backfilling a lot of them (in the daily cronjob), leading to unnecessary jobs.

To fix that, we take the updated_at value for the root step, and set it to all the steps for the same dataset.

See https://huggingface.slack.com/archives/C04L6P8KNQ5/p1708640247358519?thread_ts=1708637658.767199&cid=C04L6P8KNQ5 (internal)


set updated_at field to the same value for all steps of a dataset: After the migrations:
- `_20240221103200_cache_merge_config_split_names`
- `_20240221160700_cache_merge_split_first_rows`

the updated_at field is not accurate anymore, leading to backfilling a lot of them (in the daily cronjob), leading to unnecessary jobs.

To fix that, we take the updated_at value for the root step, and set it to all the steps for the same dataset.

See https://huggingface.slack.com/archives/C04L6P8KNQ5/p1708640247358519?thread_ts=1708637658.767199&cid=C04L6P8KNQ5 (internal)

