Use large runner for db:check-schema job

What does this MR do and why?

This MR uses a larger runner for db:check-schema (if available!) because it has started being on the critical path.

Note: This is only a hotfix by throwing money (read: doubling the runner cost for this job, it’s negligible overall but if we did this for all duration problems, we’d spend notably more money on CI) at the problem.

Related to gitlab-org/quality/engineering-productivity/team#564

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