CI: Use DNF 4 in mock to workaround dnf5#1084
    
    mock-core-configs-39.3 switched the package manager for Fedora 40 to dnf5
      https://fedoraproject.org/wiki/Changes/BuildWithDNF5
    
    dnf5 builddep cannot handle BuildRequires with square brackets,
    e.g. with Python extras, leading to errors like:
    
      No match for argument: python3dist(setuptools-scm[toml]) >= 5
      No match for argument: python3dist(raven[flask])
      No match for argument: python3dist(ini2toml[lite]) >= 0.9
    
    This is reported: https://github.com/rpm-software-management/dnf5/issues/1084
    
    Until it is fixed, we explicitly use the "dnf" (i.e. DNF 4) package manager in mock,
    even on Fedora 40+.