elixir: remember where a package is defined

A bunch of human-facing things around Nixpkgs depend on knowing where
a package was defined. This PR makes it so ofborg can ping elixir
maintainers.

pkgs/development/interpreters/elixir/generic-builder.nix

@@ -7,7 +7,7 @@
 , sha256 ? null
 , rev ? "v${version}"
 , src ? fetchFromGitHub { inherit rev sha256; owner = "elixir-lang"; repo = "elixir"; }
-}:
+} @ args:
 
 let
   inherit (stdenv.lib) getVersion versionAtLeast;
@@ -62,6 +62,7 @@ in
             --replace "/usr/bin/env elixir" "${coreutils}/bin/env elixir"
     '';
 
+    pos = builtins.unsafeGetAttrPos "sha256" args;
     meta = with stdenv.lib; {
       homepage = https://elixir-lang.org/;
       description = "A functional, meta-programming aware language built on top of the Erlang VM";