this patch adds the `services.flatpak.package` option to
allow overriding the package added by this module to
`environment.systemPackages` and the likes.
This is useful in scenarios where applications call the
flatpak binary to query information like writable directories
and there is a custom package returning different results
from the vanilla binary.
See https://github.com/crabdancing/nixpak-flatpak-wrapper
1d66aac7a0