use zsh from nixpkgs for default user shell

[WilliamHsieh]

problem

the system is loading zsh from /bin/zsh, which is set by chsh, while zsh plugins are using glibc compiled by newer version with nix

solution

1. add nixpkgs version of zsh to /etc/shells (can be done through system-manager?)
2. chsh manually for the affected user