This is a backport of electrum#7785 and electrum#10019
Depends on D19001
> build appimage: bump pkg2appimage version
>
> should fix https://github.com/spesmilo/electrum/issues/7784
> by including commit https://github.com/AppImage/pkg2appimage/commit/1e3ecde6b92ef198405463415b8090a4435d0cb0
> 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.