_artifact.py: Store sandbox config and environment in artifacts
This commit adds sandbox config and environment variables to the
artifact, so that they can be used later to shell into a downloaded
artifact or to run commands such as `bst artifact checkout --integrate`
which require running sandboxes on downloaded artifacts.
Summary:
* _artifact.py:
- Store new data in the new proto digest, similar
to how we store public data.
- Added new accessors to extract these from a loaded artifact.
- Bump the proto version number for compatibility
* _versions.py: Bump the global artifact version number
* element.py: Call Artifact.cache() with new parameters
* tests/cachekey: Updated cache key test
32 files changed
