Prevent merge conflicts when running customer tests. (#86063)

.cirrus.yml

@@ -154,7 +154,6 @@ task:
         # otherwise we won't be able to figure out how old or new our current branch is.
         - git config user.email "cirrus-bot@invalid"
         - git fetch origin master:master
-        - git rebase master
         # The actual logic is in a shell script so that it can be shared between CIs.
         - (cd dev/customer_testing/; ./ci.sh)