ci: Run dnf update in packit tests

The daily builds copr repository is enabled after installing the
packages so we need to run an update to get the latest version of
libblockdev.

plans/tests.fmf

@@ -10,6 +10,7 @@ prepare:
     script:
       - sudo dnf install -y 'dnf-command(copr)'
       - sudo dnf copr enable -y @storage/blivet-daily
+      - sudo dnf -y update
 
   - name: ansible
     how: ansible