Instructions for running projects with an older Nix version

[imincik]

Due to NixOS/nix#6820 , we can't use Nix package present in current Debian Stable, Ubuntu LTS and some non-lts Ubuntu versions as well.

• If Nix version is >= 2.24, run demo using

    nix-build /default.nix

• If Nix version < 2.24, run demo using

    NIXPKGS="https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz"
    nix-shell -I nixpkgs=$NIXPKGS --packages nix --run "nix-build /default.nix"

[fricklerhandwerk]

As @eljamm mentioned, we may want to use the exact Nixpkgs version we have in NGIpkgs to save a second download?