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.

src/lua/init.c

@@ -843,6 +843,18 @@ tarantool_lua_init(const char *tarantool_bin, int argc, char **argv)
 	}
 	luaL_openlibs(L);
 
+	/* Set _G.arg. */
+	lua_newtable(L);
+	lua_pushinteger(L, -1);
+	lua_pushstring(L, tarantool_bin);
+	lua_settable(L, -3);
+	for (int i = 0; i < argc; i++) {
+		lua_pushinteger(L, i);
+		lua_pushstring(L, argv[i]);
+		lua_settable(L, -3);
+	}
+	lua_setfield(L, LUA_GLOBALSINDEX, "arg");
+
 	/*
 	 * Create a table for storing loaded built-in modules.
 	 * Similar to _LOADED (package.loaded).
@@ -941,17 +953,6 @@ tarantool_lua_init(const char *tarantool_bin, int argc, char **argv)
 
 	luaopen_tarantool(L);
 
-	lua_newtable(L);
-	lua_pushinteger(L, -1);
-	lua_pushstring(L, tarantool_bin);
-	lua_settable(L, -3);
-	for (int i = 0; i < argc; i++) {
-		lua_pushinteger(L, i);
-		lua_pushstring(L, argv[i]);
-		lua_settable(L, -3);
-	}
-	lua_setfield(L, LUA_GLOBALSINDEX, "arg");
-
 #ifdef NDEBUG
 	/* Unload strict after boot in release mode */
 	if (luaL_dostring(L, "require('strict').off()") != 0)