Expose updated_at env var field
We currently return created_at, but not updated_at. The latter is a much more useful piece of data to return since it pertains to secret rotation/changes.
It would be ideal to return both of these values in env var data, e.g. https://circleci.com/docs/api/v2/#operation/listEnvironmentVariablesFromContext
We already have this data in our DB; it's just a matter of including it in internal and public API responses.