river: Set Adwaita xcursor theme if available

Currently we do not set any cursor theme in our river compositor. This
commit changes it to be set if the Adwaita/cursor icon directory is
available.

This does _not_ mean all applications will use Adwaita cursors
unfortunately. In Wayland applications themselves decide what to do, so
some applications do not respect the setting, e.g. GTK applications.

See the following links for more info: 

https://reddit.com/r/voidlinux/comments/wnfwy5/some_apps_not_inheriting_mouse_cursor_theme/

https://codeberg.org/river/wiki

https://codeberg.org/river/river/issues?state=all&type=all&labels=&milestone=0&project=0&assignee=0&poster=0&q=xcursor

desktop/.config/river/init

@@ -50,6 +50,10 @@ riverctl spawn "dbus-update-activation-environment SEATD_SOCK DISPLAY WAYLAND_DI
 riverctl focus-follows-cursor normal
 riverctl set-cursor-warp on-output-change
 riverctl hide-cursor when-typing enabled
+
+if [ -d "/usr/share/icons/Adwaita/cursors" ]; then
+    riverctl xcursor-theme Adwaita
+fi
 riverctl attach-mode bottom
 
 # Set background and border color