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/setup-hook.sh

@@ -0,0 +1,5 @@
+addErlLibPath() {
+    addToSearchPath ERL_LIBS $1/lib/elixir/lib
+}
+
+envHooks+=(addErlLibPath)