This is one of the things that are fixed far in the future.
In the age of 91.0, though, builders can build, repair, and assist and be able to fire.
However, reclaim and resurrect block firing and there's nothing you can do about this. Try not Fighting your commander on reclaim heavy maps (and coms dont have any smart UnitAi anyways)
Oh, also all of the above causes the builder to decloak, but resurrection doesnt.