Update `runtime_url_template` from `AppImage/AppImageKit` to `AppImage/appimagetool`

[lrh3321]

DO NOT USE ANYMORE

Please switch to the new version at
https://github.com/AppImage/appimagetool/releases/tag/continuous

This version is merely kept around for legacy compatibility. It is not recommended to be used anymore and is no longer actively supported.

https://github.com/AppImage/AppImageKit/releases/tag/continuous

appimage-builder/appimagebuilder/modules/appimage.py

Line 91 in 61c8ddd

runtime_url_template = "https://github.com/AppImage/AppImageKit/releases/download/continuous/runtime-%s"