Add untriaged performance regressions to the performance triage report

[rylev]

In order to track that performance regressions are being triaged, we should make sure that they're being listed in the triage report.

This should be possible to get from the GitHub API by searching for perf-regression and filtering all those with perf-regression-triaged. We should be able to do that directly in the API with:
https://api.github.com/search/issues?q=repo:rust-lang/rust+label:perf-regression+-label:perf-regression-triaged+state:closed

[rylev]

This was fixed in #907