Backfill tag_name on ci_runner_taggings

What does this MR do and why?

This MR backfills the tag_name column that was added to the ci_runner_taggings table in !205053 (merged).

I tried iterating by tag_id with LooseIndexScanBatchingStrategy, but it seems to perform worse (20 hours and 20 minutes) than simply going tagging by tagging (10 hours).

Changelog: other

References

• Backfill `ci_runner_taggings.tag_name` column (#570217 - closed)
• Add name column to ci_runner_taggings table (!205053 - merged)
• Add tag_name column to ci_runner_taggings table (!205435 - merged)

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