Ensure dnf-plugins-core before calling "dnf copr"

Technically dnf doesn't require dnf-plugins-core so it's possible "dnf
copr" may not work.  Our Fedora 34 images aren't pre-installing it
(something we should probably fix) but this should be fine as a
generic saftey bootstrap anyway.

Change-Id: I8a645f582f5955c93b4e115ad8bed7c46def5c82

roles/ensure-cabal/tasks/main.yaml

@@ -9,6 +9,12 @@
   register: _cabal_version
 
 - block:
+    - name: Ensure copr dnf plugin
+      package:
+        name: dnf-plugins-core
+        state: present
+      become: yes
+
     - name: Install cabal copr
       command: dnf copr enable -y petersen/cabal-install
       become: true