Prefix unstable packages with 0-

[fabaff]

With NixOS/nixpkgs#280312 the current way of nix-init setting the version of unstable packages doesn't match with the new guidelines.

[doronbehar]

Just adding here, that for projects which I want to use their latest master branch commit, while the project had a release, the version string should be ${latest-release-version}-unstable-XXXX-XX-XX.