Ported mission GDI03 to the new Lua API

Followup of https://github.com/OpenRA/OpenRA/pull/6786, https://github.com/OpenRA/OpenRA/pull/6769 and https://github.com/OpenRA/OpenRA/pull/6762.
If you skipped the delay, you could get rid of the workaround with the boolean. Is there any reason for that delay?