nixos: zsh: setopt prompt_sp to workaround a zsh bug
nixos: zsh: setopt prompt_sp to workaround a zsh bug

See #38535, properly fixing the prompt seems complicated, and this seems
to work in all the ttys I checked.

Suggested by @Mic92.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs