Cache the sandbox root when caching build trees
o _artifact.py: Load and store buildroot, added get_buildroot() API
o _artifactcache.py: Push and pull buildroot
o _stream.py: Don't require dependencies when using `--use-buildtree`, the
dependencies are no longer needed.
Also remove unused function _buildtree_pull_required()
o element.py: Use the artifact buildroot to implement shells which use
the buildtree
o tests/integration/shellbuildtrees.py: Updated test expectations, and
also now test that `--use-builtree` works without pulling dependencies
5 files changed
