gistfile1.txt

let
  pkgs = import (fetchTarball  https://nixos.org/channels/nixos-19.09/nixexprs.tar.xz) {};
  newpkgs = import pkgs.path { overlays = [ (pkgsself: pkgssuper: {
    packageOverrides = self: super: {
      erlang = super.erlangR20;
    };
  } ) ]; };

newpkgs.stdenv.mkDerivation {
  name = "my_env";
  buildInputs = [
    erlang
    elixir_1_8
    rebar3
  ];
}

I think packageOverrides is the wrong option, just replace erlang:

let
  pkgs = import (fetchTarball  https://nixos.org/channels/nixos-19.09/nixexprs.tar.xz) {};
  newpkgs = import pkgs.path {
    overlays = [ (self: super: {
      erlang = super.erlangR20;
    })];
  };

  fn = {stdenv, erlang, elixir_1_8, rebar3}: stdenv.mkDerivation {
    name = "my_env";
    buildInputs = [
      erlang
      elixir_1_8
      rebar3
    ];
  };

in newpkgs.callPackage fn {}

Replacing buildInputs = [ erlang ] with buildInputs = [ (builtins.trace erlang.version erlang) ] prints out 20.3.8.9, so I think it's doing the right thing now.