nushell: [install] Query nu for autoload directory

Nushell uses platform-native config dirs (Application Support on
macOS, AppData on Windows), so $XDG_CONFIG_HOME/nushell/autoload is
wrong outside Linux and the generated file is silently ignored.
Ask `nu` for `$nu.user-autoload-dirs` instead. Safe because the
earlier shells loop already drops `nushell` from $shells when `nu`
is not on PATH.