format_cirrus_dump: Use output snapshot id for filenames

Internally, for historical reasons, the snapshot_id is set to the airflow data_interval_start. This can be confusing for end users, as their expectation is that the date on the files aligns with the date the dump was taken. To address this we publish with a directory name that has the data_interval_end, but the files themselves are still dated with the internal snapshot_id.

Make things consistent by providing the external snapshot id to the dumps script and use that to rename the files.

Bug: T411347