linux: wayland: fix cursor set_icon (#10374)

Release Notes:

- Partially (Wayland implementation) Fixed
[#10124](https://github.com/zed-industries/zed/issues/10124)).

The recent refactor of the linux gpui implementation
(https://github.com/zed-industries/zed/pull/10227) broke the wayland
cursor update logic by setting the cursor icon as `None`. Fix it by
setting the `cursor_icon_name`.