build-init: Export an extension's runtime in the metadata
build-init: Export an extension's runtime in the metadata

Without this, extensions cannot use extra-data, as there is no
indication of what runtime to run apply_extra in.

Closes: #2954
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak