N+1 (cached) issue in merge request cached_widget.json

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<project path>/-/merge_requests/:id/cached_widget.json endpoint is periodically polled by FE for changes in MR.

We could preload artifact projects - in MergeRequests::PipelineEntity we iterate through all artifacts and call BuildArtifactEntity which uses job.project (we already preload job.artifact) - although currently this SQL query is "cached" (as it's same proejct for all artifacts), it would be better to preload artifact projects.