Cache YAML file in build environment

[ericholscher]

We will often not know much about the actual YAML file used to construct a build environment. We could in theory go back in time to find it, but we should cache it at the top level of the environment, so we can query it in the future.

[agjohnson]

I proposed storing a metadata json blob with builds that includes the configuration we used to build. This could be just the yaml file, but i think it would be more helpful as a blob of all of the settings used in the build. This would be more useful than caching for query, as we can also display metadata around the build to users -- build image used, python version used, requirements file found, etc.

[agjohnson]

Closing in favor of #3751