Snowman builds should be part of CI pipeline

We should verify that snowman builds work for every commit. This MR adds that facility to GitLab CI.

The only thing I'd like in the future, is to verify that stack builds the snowman as well, but I'm not sure how we'd integrate that into the CI machine, as it doesn't have stack installed on a global scope. Furthermore, using nix-shell -A stack --command "stack build" doesn't seem to work - I'd really like help with this.