Thanks @cjwatson for the prompt fix! My 0AD snap now builds again.
It currently uses the snapcraft from xenial-updates, which has been stuck at an old version for a while for various reasons. I believe there’s still work in progress to get that updated; separately, I have a set of branches in the review queue to let us switch over to consuming snapcraft as a snap from the stable channel. Either way, you won’t have to wait for 18.04, though I don’t quite have an ETA yet.