Only set <check_update>true</check_update> for the AppImage

[davidhedlund]

Current Behavior

Both the flatpak package and the appimage generates
<check_update>true</check_update> in settings.xml

That is valid for the AppImage, but problematic for the Flatpak package, because when you open it it asks you it you want to update Cemu, and downloads the latest AppImage if you click "Yes". This happens everytime you open the Flatpack package, even if you have clicked "Yes" previously. The only way to prevent this is to manually change <check_update>true</check_update> to <check_update>false</check_update> in ~/.var/app/info.cemu.Cemu/config/Cemu/settings.xml

Expected Behavior

As pointed out here:
This line

Cemu/src/config/CemuConfig.h

Line 416 in 2e82947

ConfigValue<bool> check_update{true};

should have a macro setting the value to true #if LINUX && !APPIMAGE