CI: Regardless of the package manager, use dnf.conf in mock config
    
    After https://github.com/rpm-software-management/mock/pull/1332
    
    We saw:
    
        ERROR: Config error: /tmp/fedora-39-x86_64-ci.cfg: '{% if releasever|int >= 40 %}dnf5{% else %}dnf{% endif %}.conf'
    
    We cannot use the templated value of package_manager in a key.
    
    The rawhide mock config uses dnf.conf anyway, despite using dnf5.
    
    Reported to mock in https://github.com/rpm-software-management/mock/issues/1357