lego fails to build in buster-slim
if we try to build the latest lego in a debian:buster-slim container image, it fails with:
RuntimeError: Failed to install local package
this is the same bug we were having in Jenkins (tpo/tpa/team#40495 (closed)), except that we can now reproduce in GitLab CI.
related: tpo/tpa/ci-templates#3 (closed)