deny warnings

3 jobs for master in 6 minutes and 11 seconds (queued for 2 seconds)
Name Stage Failure
build-coq.8.11.2 Build
make[3]: Nothing to be done for 'exercises'.
if [ -n "$(git status --porcelain)" ]; then echo 'ERROR: Exercise files are not up-to-date with solutions. `git diff` after re-making them:'; git diff; exit 1; fi

real 0m30.947s
user 2m19.200s
sys 0m5.584s
[buildjob] Aborting build because of warnings.
ERROR: Job failed: exit code 1