Some of the reasons:
- dnf doesn't support --downloaddir option
- dnf doesn't support `copy_local` repo option (used in automated tests
only)
- dnf is horribly slow, especially without cache fetched
(https://bugzilla.redhat.com/show_bug.cgi?id=1227014)
This is all needed (instead of simply using `yum` command), because
Fedora >= 22 have an command redirection `yum`->`dnf`.
QubesOS/qubes-issues#1282
