schema: builds: switch `valid` from boolean to a full-fledged status

Build status field `valid` is a boolean, which doesn't allow us to encode information if the failure happened due to the build itself or to an infrastructure error. Maestro is using a field in `misc` for that for now.

Having a full-fledged status similar to what `tests` have would solve this issue.

Start by locating the schema definitions for builds and tests, then compare how the tests status is represented with the current boolean builds.valid field. Review the Maestro-specific value currently stored in misc. Done means builds can distinguish build failures from infrastructure errors using a full-fledged status without relying on misc.

Written by the indexing model from the issue text.