Put runner machine heartbeat under a feature flag

What does this MR do and why?

Related to gitlab-com/gl-infra/production#8547 (comment 1315879639)

It disables the runner machine heartbeat since loading it through the job relation is sub-optimal:

• https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/16513/commands/56603
• https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/16513/commands/56606
• https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/16513/commands/56609

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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