Screenshots are not cached

[danirabbit]

It doesn't look like we're generating screenshots for pushed apps?

https://flatpak.elementary.io/repo/screenshots/

[danirabbit]

Changing the title here. Looks like screenshots not appearing at all is an AppCenter issue. However, we still need to be caching screenshots so that developers can't change them on the fly, delete them, and so we can make sure that modified time is available to avoid re-downloading them