Add nix build for reference

[kpcyrd]

At the time of writing, Nix flakes are an experimental feature, they can be enabled with:

mkdir -p ~/.config/nix/
echo 'experimental-features = nix-command flakes' >> ~/.config/nix/nix.conf

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This is supposed to demonstrate how repro-env.toml compares to flake.nix, and how repro-env.lock compares to flake.lock.

To cater to the same use-case (statically linked, reproducible binaries that can be attached to your github releases), this nix.flake file needs still to be modified (it currently dynamically links with shared objects in /nix/store/..., which won't work if you just download the binary from github on some generic Linux computer) This is now working.

The flake.nix file for this project (and the flake.lock fwiw) should be as clear and simple as possible, sticking to official, reputable resources only (like eg github.com/nixos/nixpkgs). If you have any suggestions on how it can be improved, please let me know!