v6.5

• Recommend using images from Flathub instead of building our own. They have been failing to build for a long time and as Flathub is the distributor of the various SDKs, it just makes more sense for them to provide the developer-friendly containers/images to be used on CI as well.
• Update path to the mirrored screenshots
• Include '.flatpak' suffix in artifact name to avoid conflicts with other artifacts
• Fix cache to restore cache from correct arch