ci: add -Werror to CFLAGS, added clang build target
This is a followup for https://gitlab.labs.nic.cz/knot/knot-resolver/commit/a1c56f8f76270f45bd8a7ad843a7f27989ce70b5#note_65758
This MR adds stricter CI builds to prevent merges with common QA issues.
- Added -Werror to build phase
- Added a build step with clang (with no artifacts)