Wrap ActionResults in ExecuteResponses before putting them in Operations

Context

The "Operations" section of the Remote Execution API design document says that when a build job is complete, its Operation's result field must be set to an ExecuteResponse message. However, BuildGrid currently sets it to an ActionResult instead.

Task Description

Currently, the ActionResult received from the worker is passed unmodified from BotsInterface.update_bot_session to Scheduler.update_lease to Scheduler.job_complete, where it is stored in Job.result and used in Job.get_operation.

Somewhere in that chain of events (probably either in Scheduler.update_lease or Job.get_operation), it should be wrapped in an ExecuteResponse.