Why
===
* flake.nix had a lot of almost redundant lines in it.
* i686 support is probably being dropped soon in nixpkgs
https://github.com/NixOS/nixpkgs/pull/266164
What changed
==
* Add new input nix-systems/default which represents the default
systems typically used in the Nix community
* Define and use an eachSystem function that simplifies the flake.nix file
flake.lock previously included a "indirect" reference to nixpkgs.
I am not sure what this means but I added `inputs.nixpkgs` and updated nixpkgs because this old version of nixpkgs didn't have any support for aarch64-darwin at all.
Now on a aarch64-darwin I can type `nix build` and get a working version of agenix.