Sets the name of the desktop that the application is running in.
do not use this API. Since 2.42 the value of the `XDG_CURRENT_DESKTOP` environment variable will be used.
This is used by [method@Gio.AppInfo.should_show] and [method@GioUnix.DesktopAppInfo.get_show_in] to evaluate the `OnlyShowIn` and `NotShowIn` keys.
Should be called only once; subsequent calls are ignored.
| desktop_env |
a string specifying what desktop this is |