Moves the final step for building the AppImage to the upload script.
Instructs appimagetool to embed update information into the AppImage if
the release target is Mainline. Also tells it to create a zsync file to
enable partial-downloads when updating the AppImage.

Also renames the AppImage from `yuzu-{version info}-x86_64.AppImage` to
`yuzu-{version info}.AppImage` to avoid a bug in the downloads page at
yuzu-emu.org/downloads.