Previously we would use one builder on Travis to produce two sets of host
compilers for two different targets. Unfortunately though we've recently
increased how much we're building for each target so this is starting to take
unnecessarily long (#40804). This commit splits the dist builders in two by
ensuring that we only dist one target on each builder, which should take a much
shorter amount of time. This should also unblock other work such as landing the
RLS (#40584).