RHEL7: "leapp preupgrade" fails with message "Failed to synchronize cache for repo"

Issue

• leapp preupgrade fails with below error when system is registered to Red Hat customer portal using RHSM:

  Unable to install RHEL 8 userspace packages.
  'dnf', 'install', '-y', '--nogpgcheck', '--setopt=module_platform_id=platform:el8', '--setopt=keepcache=1', '--releasever', u'8.4', '--installroot', '/el8target', '--disablerepo', '*', '--enablerepo', u'rhel-8-for-x86_64-baseos-rpms', '--enablerepo', u'rhel-8-for-x86_64-appstream-rpms', 'dnf'] failed with exit code 1.
          Failed to synchronize cache for repo 'rhel-8-for-x86_64-appstream-rpms', ignoring this repo.
          Failed to synchronize cache for repo 'rhel-8-for-x86_64-baseos-rpms', ignoring this repo.
          Error: Unable to find a match: dnf