cr-buildbucket.cfg: Tighten bot dimensions
cr-buildbucket.cfg: Tighten bot dimensions

There was flakiness on CI because of two things:
 - On Windows there is a bot that's only 32bit and our tools are 64bit
so the gn gen step was failing.
 - On macOS we require a minimum SDK version of 10.12 (the default in
build/'s GN files) and would get scheduled on macOS 10.9.

Change-Id: I47db92ec111d310d9b8cf8d9ee75a415df021581

googlearchive/nxt-standalone,googlearchive/nxt-standalone,googlearchive/nxt-standalone,googlearchive/nxt-standalone