Add nix flake for building

[oscar-schwarz]

I had a hard time figuring out how to build this with nix (my laptop has the 55b4 reader) to add this to my nix config. Adding a flake.nix makes building a lot easier. Sadly I wasn't able to make the unit tests work (I didn't want to mess with the code). Most of the nix package is copied from: https://github.com/NixOS/nixpkgs/blob/88a55dffa4d44d294c74c298daf75824dc0aafb5/pkgs/by-name/li/libfprint/package.nix#L92 (The current implementation of the original libfprint in nixpkgs)