The latest Debian bullseye release doesn't provide yum any more, only
DNF. This breaks the minimal builds that are using on-host yum tools
to start the chroot. Probe for yumdownloader, and if it's not there,
use DNF.
Note this requires "dnf download" which may not be packaged. See
I21cfbd3935e48be4b92591ea36c7eed301230753 for a sample work-around
that installs this plugin in the nodepool-builder container.
Change-Id: Ia7f1e4d115cc67c378d865d91af94a07b8cdc6cc
