Move `demo-*` builds to **only** system packages

The `demo-*` builds install gRPC and Protobuf because some libraries do not compile with the versions packaged with the underlying distro.

We should disable the libraries that do not work. It is probably more important to show how customers can use the system packages for our dependencies than to show how to compile the latest and greatest version.

We can add another builds that builds everything from source if we think it is important to show how that can be done.

Start by locating the demo-* build definitions and how they select gRPC and Protobuf dependencies. Identify the libraries that do not compile with the underlying distribution packages, then verify that the demos use only system packages and disable those libraries. No specific files or tests are named in the issue.

Written by the indexing model from the issue text.