console: fix lang identification in local_read()

Language in `local_read()` used to be set to `box.session.language` while
the latter is always `nil` and `set_language()` sets `self.language`.

Now the language in `local_read()` is identified correctly.

Needed for tarantool#4317

NO_DOC=bugfix
NO_CHANGELOG=invisible to user
NO_TEST=invisible to user

src/box/lua/console.lua

@@ -649,7 +649,7 @@ local function local_read(self)
             break
         end
         if delim == "" then
-            local lang = box.session.language
+            local lang = self.language
             if not lang or lang == 'lua' then
                 -- stop once a complete Lua statement is entered
                 if local_check_lua(buf) then