Introduce a --build-tools=next flag to subsume --ninja=next

We currently offer a flag that allows us to do a staged rollout of updates to the Ninja builder, under the flag -ninja=[stable|next]. However, there are other opt-in tools like ccache.py and icecream.py where it would make sense to do similar staged rollouts. Introduce a flag -build-tools=[stable|next] to allow the whole suite of optional tools to be staged.