Changes in 1.6.4

This release backports some of the OCI authenticator fixes from the 1.7 series, and
should now be able to host flatpak images on e.g. docker hub.

Other changes:

• Fix a use-after free in libflatpak
• Don't list p2p downgrades in list of available updates

$ sha256sum flatpak-1.6.4.tar.xz 
700328b990d89a21f27b7365611a1e51879916cd69068b0bf794df057c9fe448  flatpak-1.6.4.tar.xz