Support building with nix

[Mic92]

I just want to drop this expression here, that allows to build solo5 with nix:

with import <nixpkgs> {};
stdenv.mkDerivation {
  name = "env";
  nativeBuildInputs = [
    bashInteractive
    pkg-config
  ];
  hardeningEnable = [ "pie" ];
  buildInputs = [
    libseccomp
  ];
}

Also needs https://github.com/Solo5/solo5/pull/496