Updates the appimage build scripts to use the newer
https://github.com/AppImage/appimagetool tool to bundle the appimage
instead of the discontinued https://github.com/AppImage/AppImageKit.
To prevent the new appimagetool from downloading a random "latest"
appimage runtime (`type2-runtime`) binary this PR also adds
functionality to clone and build
https://github.com/AppImage/type2-runtime from source. This is done
using the build scripts provided in the `type2-runtime` repository,
however the Dockerfile they use for building is replaced by a copy with
pinned package versions to prevent issues with reproducibility.
This should fix the issue of missing libfuse2 which users of the appimage
have on "modern" distributions.
The new `type2-runtime` is statically linked and includes the required
dependencies now instead of relying on the host to provide it.
b93ffdd79d