You can't just `nix-channel --update && nix-env -uA nixpkgs.nix` to get it -- it's apparently not on the master branch. Funny for a release. But here's how you get it:

- Check out the nix-2.0 branch of github:nixos/nixpkgs
- nix-env -f . -iA nix

This will install nix-2.0pre5914_48c192ca, which also sounds funny for a release.