Fixed captured C&C command centers giving no new build options

As described in #2129 and #2187 capturing another factions construction yard or vehicle factory gave no new build options but your own while in the originals you could steal technology. This fixes it for the Tiberian Dawn mod which turned out to be more work than I anticipated. There is still a bug that, the tech tree does not auto-update on capture so you need to redeploy the MCV, but then it works.
Maybe change this comment to reflect what is actually happening now? It's true that after capture it will contain the tech of the original owner, but the InitTech method will also update what is producable based on the new owner's currently built buildings.  Maybe ```Update producable to contain the tech from the original owner, based on the new owner's currently built tech.```  Which is what has changed to make this work.