testsuite images on aarch64 needs C++ compiler

grpcio has to be compiled from source on aarch64 so we need to install a C++ compiler there. At present, we install it during the test run on BuildStream's CI but ideally they would be provided by this repo.

This means that our yaml logic will require some more thought as we will now have to support packages that are specific to certain architectures.

See buildstream!1065 (comment 131341848) for some related discussion.