[Building] Allow prioritizing package builds on flathub based on whether they build from source or prebuilt binaries

[orowith2os]

I don't think Flathub currently has any priority checking of application builds, however I think it would be ideal to allow specifying package bases inside of an application's flathub.json.

This would allow for getting binary-based builds done faster, or source-based ones, depending on how Flathub wants to handle it.