Resolve "gitlab-backup is not fully utilising environment variable overrides"

What does this MR do and why?

Decreases the batch size for iterating over group wiki repositories during repository backup via the gitlab:backup:create task.

For installations with a large number of group wikis, and using pgbouncer, the database connection can timeout while iterating over the groups, causing the backup to fail.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

It is complicated to recreate the underlying issue, but at a minimum, this change should not break backups

1. Add group wikis to a GDK installation
2. bin/rail gitlab:backup:create should succeed
3. Checkout this branch
4. bin/rail gitlab:backup:create should still succeed

Related to #464146 (closed)