The idea behind that is to enable users and developers of
downstream tools such as home-manager to test Nix master for several
reasons:
* Nix is currently trying to have a `master` branch that's always
releasable[1]. We're still on Nix 2.18 in nixpkgs due to too many
notable regressions. Enabling people to test latest master may help on
that end.
* This uses the most bleeding-edge Nix, but our packaging, so we can
identify issues with our packaging early.
* From what I've seen, most people are using the packages from nixpkgs
anyways instead of the upstream flake, this is far more convenient
anyways.
My plan is to update this once a week. Right now we rely on the
`installCheckPhase` here, but as soon as we have proper regression
testing[2], we may want to add `nixUnstable` there as well (however with
failures being allowed probably).
[1] https://discourse.nixos.org/t/nix-release-schedule-and-roadmap/14204
[2] https://github.com/NixOS/nixpkgs/pull/304332
