Polish and use native workspaces in executors mode
Created by: eseliger
This PR cleans up the src side of things for SSBC:
- Ensures to have a separate cache directory for each src running, so they don't collide while writing to them
- Ensures to have a tmp directory set within the executor workspace so it can be retained for inspection
- Removes useless API call to Sourcegraph to retrieve it's version
- Supports native repos cloned into the workspace
- Ensures no env var is used for cache key generation, those are not set on the server and it would cause cache key mismatches
- And most importantly, it doesn't require an access token anymore, so it's much safer, especially in less-isolated executors!
Closes https://github.com/sourcegraph/sourcegraph/issues/37079 Closes https://github.com/sourcegraph/sourcegraph/issues/36597 Closes https://github.com/sourcegraph/sourcegraph/issues/36187
Test plan
Lots of manual validation. I will also shove it into k8s post-merge and see if it behaves well.