Prioritize CI mirrors

What does this MR do?

By setting the next execution time in the past, they will be prioritized before any other mirrors.

This is already applied to a hotpatch in production.

https://gitlab.com/gitlab-org/gitlab-ee/issues/10844

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content