make-koji-release-tags: Set %distcore for flatpak build tags
    
    Previously, we were setting the .dist macro in flatpak-runtime-config;
    do it in the build tag configuration to match ELN and for greater
    flexibility: in particular this allows
    
    This depends on a fedora-release change to define %dist in terms of
    %distcore:
    https://src.fedoraproject.org/rpms/fedora-release/pull-request/275