lua: enable/disable override loader

A script for preparing tarantool for executing LuaJIT submodule test
suites has a `debug.getupvalue()` call. It is fragile and the previous
commit promises that it'll be rewritten. It is done here.

The override loader can be disabled using an internal API or by the
`TT_OVERRIDE_BUILTIN` environment variable. The latter is not used
anywhere at the moment of writing and added just in case.

Follows up #7774

NO_CHANGELOG=not planned to be a documented feature, just a tiny
             workaround for specific tests
NO_DOC=see NO_CHANGELOG