(Spin off of #9900)

We currently test Nix (bot in GHA and Hydra) by building its flake, which uses a pinned version of the latest stable NixOS release.
This means that:

1. We never test that Nix works properly in nixos-unstable (although a large part of the recent Nix releases are probably following that channel since the stable releases freeze the version of Nix they embed)
2. Upgrading to new NixOS releases can be a pain (see ~Add nixos-unstable based outputs, bdwgc 8.2.4~, Nixpkgs lib tests, cleanup #9900 which tool 3 months overall)

To limit these problems, we need to also test that Nix works fine when built from nixos-unstable, which means running the test suite in the CI (either hydra or github actions) with everything built from unstable.