Detect erlang ERTS directory at runtime.

Index: bin/elixir
--- bin/elixir.orig
+++ bin/elixir
@@ -225,7 +225,9 @@ fi
 
 # One MAY change ERTS_BIN= but you MUST NOT change
 # ERTS_BIN=$ERTS_BIN as it is handled by Elixir releases.
-ERTS_BIN=
+for obsd_erts in ${LOCALBASE}/lib/erlang${MODERL_VERSION}/erts-*; do
+  ERTS_BIN="$obsd_erts/bin/";
+done
 ERTS_BIN="$ERTS_BIN"
 
 set -- "$ERTS_BIN$ERL_EXEC" -noshell -elixir_root "$SCRIPT_PATH"/../lib -pa "$SCRIPT_PATH"/../lib/elixir/ebin $ELIXIR_ERL_OPTIONS -s elixir start_$MODE $ERL "$@"