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.

install

@@ -10,7 +10,6 @@ shells="bash zsh fish nushell"
 prefix='~/.fzf'
 prefix_expand=~/.fzf
 fish_dir=${XDG_CONFIG_HOME:-$HOME/.config}/fish
-nushell_autoload_dir=${XDG_CONFIG_HOME:-$HOME/.config}/nushell/autoload
 
 help() {
   cat << EOF
@@ -446,6 +445,7 @@ fi
 if [[ "$shells" =~ nushell ]]; then
   if [[ $key_bindings -eq 1 || $auto_completion -eq 1 ]]; then
     echo "Setting up Nushell integration ..."
+    nushell_autoload_dir=$(nu -c '$nu.user-autoload-dirs | first')
     mkdir -p "$nushell_autoload_dir"
     dest="$nushell_autoload_dir/_fzf_integration.nu"
     echo -n "  Generate $dest ... "