Append -flatpak suffix to koji package name for flatpaks
At this point, all new flatpak requests should use the new flatpak
infrastructure, which still uses flatpaks/$NAME dist-git but
$NAME-flatpak koji "package".
Resolves: https://pagure.io/releng/issue/11626
Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>