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/modules/programs/zsh/zsh.nix

@@ -70,7 +70,7 @@ in
       promptInit = mkOption {
         default = ''
           if [ "$TERM" != dumb ]; then
-              autoload -U promptinit && promptinit && prompt walters
+              autoload -U promptinit && promptinit && prompt walters && setopt prompt_sp
           fi
         '';
         description = ''