Change daily builder

Bunch of things I've been thinking about, capturing them here for discussion. Currently we spend $85/month on the builder for 111 Hrs. We could:

* Use a faster builder (currently using c5d.4xlarge)
* Use ephemeral builders, one per build we want to do (instead of reusing a stopped builder serially, fire up a builder to do a particular piece of work, then terminate it)
* Use spot pricing to (probably radically) reduce costs