Rework the flatpak to use extra-data for the .deb
As the App requirements for Flathub state:
All content hosted on Flathub must allow legal redistribution, and the license must be correctly specified in the app's appdata file. Non-redistributable files can be downloaded at install time using the extra-data source type.
As parsec is provided under a proprietary license, the extra-data source type has to be used. This means the app needs to be reworked to no longer download the .deb
during the build but during install.