Alright, the RC is up for the snap, at least for amd64 and i386 (the other architectures are still waiting on the build farm to get meltdown/spectre mitigations). You can check it out from the
$ sudo snap install nextcloud --channel=13/candidate
Or if you already have it installed:
$ sudo refresh nextcloud --channel=13/candidate
Note that downgrades are not supported without a revert, so use your own best judgment.