[ examples] fix loading jsonl with load dataset in run translation example (#26924)
* Renamed variable extension to builder_name
* If builder name is jsonl change to json to align with load_datasets
* Apply suggestions from code review
Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>
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Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>