Nix generic builder

If we don’t provide an explicit Nix builder script, we can instead use the buildInputs attribute on mkDerivation to pass in derivations we need in scope, which are then passed into

source $stdenv/setup

buildInputs will be picked up by the setup script, adding bin directories to the PATH, lib directories to the library search path, etc. genericBuild smartly attempts to unpack and build sources as we did explicitly in our builder script.