nix develop nixpkgs#hello
    inside the shell, you can call:
        - unpackPhase
        - configurePhase
        - buildPhase

{...
    devShell.x86_64-linux =
        pkgs.mkShell { buildInputs = [ self.packages.x86_64-linux.hello pkgs.cowsay ]; };
}

2021
https://serokell.io/blog/practical-nix-flakes