Don't bundle src-cli in executors

Created by: eseliger

Currently, to get a new and improved src-cli version to executors, we need to run a full executors release and rollout. This feels inefficient and limits our ability to act on fixes. Also, an executor must match the Sourcegraph version exactly as of today, because src-cli needs to have the feature set required by the instance. If we can get the executors API stable across multiple releases, this would be the last blocker for easing the version constraints.

For this, we might want to bundle the src-cli release in the SG distribution, and fetch the latest supported version on the go. Exact details TBD, and security input would be required here as well. This has to work on air-gapped instances as well.