dom0-updates: use dnf --best --allowerasing

Otherwise `dnf install` do not want to upgrade existing packages, or
upgrading other packages to satisfy dependencies.

Fixes QubesOS/qubes-issues#2100

misc/qubes-download-dom0-updates.sh

@@ -53,7 +53,7 @@ fi
 
 YUM="yum"
 if type dnf >/dev/null 2>&1; then
-    YUM="dnf"
+    YUM="dnf --best --allowerasing"
 fi
 
 if ! [ -d "$DOM0_UPDATES_DIR" ]; then