test_k8s/dumps: rely on the dagrun data interval end date instead of start date to craft folder names

• start_date determines when scheduling begins, but doesn't directly determine when tasks execute
• data_interval_date identifies the end of the data period being processed

As we were using start_date instead of data_interval_date, we wouldn't craft the right date folder when restarting one or multiple days after the task scheduling.

Signed-off-by: Balthazar Rouberol brouberol@wikimedia.org Bug: T388378

Closes T388378