Preload runner manager in :id/pipelines/:pipeline_id/jobs

What does this MR do and why?

This MR preloads :runner_manager for ci_builds in the :id/pipelines/:pipeline_id/jobs endpoint, to fix an N+1 test that was not detected when the !151679 (merged) Community contribution was merged.

Basically, the original MR added runner_manager to the Job entity, which caused one more table join. However, the full pipeline run didn't detect this failure and the MR was merged, causing gitlab-org/quality/engineering-productivity/master-broken-incidents#6277 (closed).

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