run deadnix during `nix flake check`

[adrian-gierakowski]

to make sure there is no unused binding etc
https://github.com/astro/deadnix

it can be added to treefmt as it's done here: https://github.com/NixOS/flake-compat/blob/5edf11c44bc78a0d334f6334cdaf7d60d732daab/dev/config.nix#L35-L38