Imports code.proto from https://github.com/googleapis/googleapis and
corresponding modules generated with ./setup.py build_grpc.

#668

#668

Better handle remote build failures

See merge request !825

Unsupported sandbox config (UID/GID) is now reported by the element.

On systems without user namespace support, elements with unsupported
sandbox config (UID/GID) are now individually tainted, which disables
artifact push.

The artifact cache is no longer platform-specific.

Now that the platform is independent of the context, explicit
instantiation is no longer required. This avoids issues with platform
instances used across test cases with mismatching context.