mix.bbclass: Fix mix release erts path
mix.bbclass: Fix mix release erts path

Without this change building a mix release does not work on meta-erlang
master, since the erts path is wrong. The directory erts-<version> is
now appended to it.
Tested with erlang 23.2.1 and elixir 1.11.2

joaohf/meta-erlang,joaohf/meta-erlang