feat: manage global zsh on nix-darwin

ty @NyxKrage

machines/sashimi/default.nix

@@ -12,6 +12,8 @@
     hostName = name;
   };
 
+  programs.zsh.enable = true;
+
   # Used for backwards compatibility, please read the changelog before changing.
   # $ darwin-rebuild changelog
   system.stateVersion = 4;