elixir: set ERL_LIBS for elixir apps

Previously Elixir didn't correct set the ERL_LIBS environment
variable for the OTP Applications that it supplies. This
change fixes that

pkgs/development/interpreters/elixir/default.nix

···
   LANG = "en_US.UTF-8";
   LC_TYPE = "en_US.UTF-8";
 
+  setupHook = ./setup-hook.sh;
+
   buildFlags = if debugInfo
    then "ERL_COMPILER_OPTIONS=debug_info"
    else "";

pkgs/development/interpreters/elixir/setup-hook.sh

···
+addErlLibPath() {
+    addToSearchPath ERL_LIBS $1/lib/elixir/lib
+}
+
+envHooks+=(addErlLibPath)