Consider making unrolled up artifacts *only* after perf regression

[rylev]

After #1372 and follow ups, we now kick off builds for all rolled up PRs in a roll up as if they were merged directly and independently into master. This works well for our purposes but is arguably a bit of a waste of resources. We may want to consider instead waiting for perf results to come back and only kick off this process if the roll up exhibits performance regressions.

[Mark-Simulacrum]

So I think there's actually a lot of benefits modulo perf to these artifacts existing (e.g., bisection), so I'd rather we keep them at least for now. We can reconsider if there's queueing on rust-lang-ci that makes us worry, but I don't expect there to be.

[rylev]

These have proved useful enough outside of perf runs that, as @Mark-Simulacrum suggests, we should continue to always build them. Closing this issue.