Don't keep old failed builds in the build queue

[syphar]

We have two main kinds of build-errors:

1. "standard" rustc build errors, which always ended up in the proper tables
2. "errors before the build even starts" (like, a broken cargo.toml file, fails when fetching dependencies).

Since we also add in-progress builds to the database, we can see both error types in the proper interfaces.

But (My guess) from the times before that, we wanted some record of the failed build. And these were these queue entries where we tried more than X times.

This leads to around ~5k entries in the queue table of old failed builds.

I don't think we need these any more and can keep a clean queue table without any issues.

I also fixed my TestMetrics / CollectedMetrics testing stub, which returned the wrong values when we queried the values multiple times.

[syphar]

@GuillaumeGomez ready for review now

[GuillaumeGomez]

Looks good to me, thanks!