If we leave files sitting around, we trigger a Docker image double-build in CI,
because the build runs once, gets tested, and then gets run again when we push
the Docker image: the additional btest files cause Docker to detect a different
source tree, causing an image layer violation.
Also rename "cleanup" target to "clean", to align with rest of our tree.
We missed the fact that the broker module didn't load due to a wrong libpython
version because we didn't test that load.
Also remove an unused environment variable from btest.cfg.
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).
This patch adds a minimal Zeek environment packaged as a container.
Since this is intended both as a base layer for other images and as a
quick way to explore Zeek we install only zeek and zkg as basic
functionality.
Closes#1625.
