appimage build on github actions - potential issue

linuxdeployqt will not allow building of appimages with ubuntu versions above 20.04 till 20.04 is out of support, which is end of May, 2025.

 But github actions has already started "brownout" of 20.04 - The Ubuntu-20.04 brownout takes place from 2025-02-01. For more details, see https://github.com/actions/runner-images/issues/11101

A workaround would be to use go-appimage instead, which does not have this hard limit on OS version. As was used for OpenSpace-AppImage.