This patch replaces the container-strucuture-tests for the Zeek
container with a BTest-driven approach to provide a more familiar
experience. In addtion to `python3` for `btest` we also rely on `make`
being available in Github-hosted runners which at least currently seems
to be hold (probably pulled in as a dependency of CMake which is
officially included, see
https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md).
