flatpak: Honor base runtime

Flatpak manifest https://manpages.debian.org/testing/flatpak-builder/flatpak-manifest.5.en.html document base and base-version to extend a runtime.

At least I'm aware of Elementary using it

But seems download is failing because it is looking for

21:25:45.0396              gbp-flatpak-runtime-provider[ 1017904]:    DEBUG: Transfer failed: No such ref 'runtime/io.elementary.BaseApp/x86_64/juno-19.08' in remote flathub

I think it should be io.elementary.BaseApp/x86_64/juno-19.08

NOTE: There're other properties to include (runtime-extensions, base-extensions) if wanted.

https://github.com/akiraux/Akira/ can be used to test this.