Making palette.json part of the system closure will protect it from
garbage collection, so future configurations can be evaluated without
having to generate the palette again. The generator is not kept, only the
palette which came from it, so this uses very little disk space.
As evaluations will no longer need the palette generator unless the
wallpaper was changed, I've removed the recommendation to add a binary
cache - the overhead of querying the cache for unrelated builds
outweighs the remaining benefits of it.