Move pycobertura installation into a pre-run

Change-Id: I850b17473f8532e09b6ece4d8f8debb490f7ff59
diff --git a/zuul.yaml b/zuul.yaml
index dfe4c6e..cf2f5a6 100644
--- a/zuul.yaml
+++ b/zuul.yaml
@@ -44,7 +44,9 @@
     description: |
       Compare difference in coverage as introduced by this change.
     parent: f29-gcc
-    pre-run: playbooks/checkout-original-state/run.yaml
+    pre-run:
+      - playbooks/checkout-original-state/run.yaml
+      - playbooks/coverage-diff/pre.yaml
     run:
       - playbooks/run-test-command/run.yaml
       - playbooks/coverage-diff/generate.yaml