.gitlab-ci.yml: Follow latest BuildStream Docker image

This allows us to remove env vars that are now set in the image, and some packages which are already installed in the image.

I've left Sphinx (which is needed for the documentation task) out of the image as it pulls in a bunch of Python 2 dependencies that would bloat the image. Although note that Python 2 is already present in the image because Bazaar depends on it.

Fixes #100 (closed)