lua: define _G.arg earlier

We're going to provide an ability to override a built-in module. The
code of an override module will work in a bit unusual circumstances: a
part of tarantool's Lua runtime is initialized, but a part isn't.

However, nothing prevents us from providing the `arg` global variable at
this stage. It used to be accessible anywhere in a user defined code.
Let's keep this property by making it accessible from an override
module.

Part of #7774

NO_TEST=It will be tested as part of the override feature.
NO_CHANGELOG=It is not possible to execute any user provided code at
             this loading stage until modules overriding will be
             implemented. So this commit doesn't change any behavior
             that a user might observe before this commit.
NO_DOC=It spreads exiting runtime guarantee to a new stage, where a user
       defined code may appear. Nothing new is introduced.