Fix and reschedule background migration for backfilling data from `ci_builds` to `security_scans`

What does this MR do?

This mr will fix bg migration (first introduced in this mr !66963 (merged)) for backfilling data from ci_builds to security_scans noticed here and reschedules the bg migration again. Thanks @pbair.

Migrations:

Up

== 20210816183304 ScheduleCopyCiBuildsColumnsToSecurityScans2: migrating ======
-- Scheduled 1 CopyCiBuildsColumnsToSecurityScans jobs with a maximum of 5000 records per batch and an interval of 120 seconds.

The migration is expected to take at least 120 seconds. Expect all jobs to have completed after 2021-08-16 18:36:09 UTC."
== 20210816183304 ScheduleCopyCiBuildsColumnsToSecurityScans2: migrated (0.0369s) 

Down

== 20210816183304 ScheduleCopyCiBuildsColumnsToSecurityScans2: reverting ======
== 20210816183304 ScheduleCopyCiBuildsColumnsToSecurityScans2: reverted (0.0000s)

Related to #333414 (closed) #336198 (closed)