Review prs on the nix-unstable channel

[wmertens]

We're getting false negatives when master is too different from hydra.

Instead, the review could be done from the nix-unstable channel branch so that the only builds are due to the PR.

In https://github.com/NixOS/nixpkgs/pull/11254 you can see how to do it, it is only a few commands.

[madjar]

It's an excellent idea! I'll look into it when I can.