Fix N+1 query in /api/v4/projects/:id/jobs endpoint

What does this MR do and why?

Fix N+1 query in /api/v4/projects/:id/jobs endpoint.

Changelog: fixed

Closes #461550 (closed)

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I think we can rely on the tests here. Previously, the tests weren't failing even if we removed :runner from the preload call, but I've fixed that.