Reschedule finding_id backfilling

What does this MR do and why?

In #418971 (comment 1576891463) we figured out there's an issue with our QA scripts which didn't populate finding_id so we need to rerun the background migration to backfill those values.

Related to #418971 (closed)

Database review

According to #418971 (comment 1576891463) we have 2304 records with finding_id = NULL so it should be reasonably quick to finish the backfill

MR acceptance checklist

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