I would do something like
```
writeScript "detect-ssh-askpass" ''
${lib.optionalString config.programs.seahorse.enable ''
if [[ $desktop = "gnome" ]]; then
      exec "${pkgs.gnome3.seahorse}/libexec/seahorse/ssh-askpass"
fi
''}
${lib.optionalString xserverCfg.desktopManager.plasma5.enable ''
if [[ $desktop = "plasma" ]]; then
      exec "${pkgs.plasma5.ksshaskpass.out}/bin/ksshaskpass" "$@"
fi
''}
''