Further stability improvements

We have been working hard to make Depot more stable and reliable. We have been running Depot in production for a few months now and have been able to identify and resolve several issues. The one we have been working on the most is the stability of builds across cloud providers. Today, we support image builds for Intel and Arm architectures by routing builds for each given architecture to their respective cloud provider (AWS for Arm and Fly for Intel).

However, this creates a coupling to cloud providers that isn't ideal for operating our remote builders at scale. The solution we have in beta currently is to route builds to different cloud providers based on outages at our existing ones, capacity restrictions, etc. This is a much more robust solution that allows us to always be ready to process a build without interruption.