Disallow warnings on `nix flake check`

I’d like to disallow warnings on nix flake check. To be clear, that would mean non-zero exit on any warnings. Any way to do that at the moment, please?

For this ngipkgs issue.

For the record, it looks like @lorenzleutgeb figured this out in ci: Set environment `NIX_ABORT_ON_WARN=true` by lorenzleutgeb · Pull Request #191 · ngi-nix/ngipkgs · GitHub