Reduce manual garbage collection frequency
In gitlab!97490 (comment 1095424902), a user sees the runner consuming 10% CPU even though there are no builds to process:
It appears this might be due to the manual garbage collection in the loop: https://gitlab.com/gitlab-org/gitlab-runner/-/blob/b2dd0a2db062f01ed7b5c724e5416e84eef7605d/commands/multi.go#L555
It seems to me that we should consider:
- Dropping that manual GC.
- Only run GC if a build was actually processed.