I made sort of a “personal hydra” this weekend using github actions + cachix. The purpose is to rebuild a subset of packages that I care about from nixpkgs-master every few hours, caching both build successes and failures.
It’s super hacky.
If anyone has a better version of this, or would like to use it themselves, let me know or just go for it.
Code is here: https://github.com/rmcgibbo/mydra
Display is here: mydra
pretty nice, might be improved by filtering by build status, as you’re mostly concerned about failing builds
Really cool 
I’ve opened https://github.com/NixOS/nix/issues/4127 a while ago since I think it would help the scenario where a long-running job is known to be failing and caching failed builds would help save ton of time.
Thanks guys. Both are good ideas.
I did use code from @timokau’s https://github.com/timokau/nix-bisect to implement negative caching, but it’s a bit of a hassle.
Another idea would be to have a similar tool automatically run @Mic92’s nixpkgs-review against new PRs on github-actions and post the results to the PR. Maybe I’ll do that next weekend. Or has anyone done that before?
Make sure to filter out mass rebuilds and if you want long running builds which is possible with a PR of mine. I can provide you with a list of packages that take a long time. If you want any feature or find bugs let me know and I can try to fix them if you don’t find the time or are not familiar with the code.