Draft: Add reference from ci_builds_metadata to ci_runner_machines

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds a FK reference from ci_builds_metadata.runner_machine_id to ci_runner_machines.id, as per https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/#runner-identification-in-ci-jobs.

Closes #386755 (closed)

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