Cannot enqueue more than one build for a PR at a time

[rylev]

When testing a regressed rollup, we may want to enqueue more than one build at a time. However, this is not possible because of a constraint on the database (found here). It's not clear to me if this constraint is actually needed or not. I can see how it might prevent certain bugs where we overly commit try builds, but in this case it is too restrictive. Can we remove this constraint?

[Mark-Simulacrum]

I believe the goal with the constraint was to deduplicate queueing, to make rust-timer queue essentially idempotent. I don't think it needs to be the case when directly specifying a SHA to build via rust-timer build, though.

I'd need to put some thought into quite how we manage that from a DB perspective, but it's also plausible we could just drop the constraint entirely. I think it was mostly intended to prevent needing to decide which row to attach to when we have a build ready to start...