base: do not fail when the prune entry is root's only cron line

grep -v exits 1 when it prints nothing, and with pipefail that failed
the task on both hosts, although the crontab had been rewritten
correctly. Ignore grep's exit status.

Assisted-by: Claude:fable-5-2

ansible/roles/base/tasks/main.yaml

@@ -70,7 +70,7 @@
 - name: Drop the old prune entry from root's crontab
   ansible.builtin.shell: |
     set -o pipefail
-    crontab -l | grep -v 'docker image prune' | crontab -
+    crontab -l | { grep -v 'docker image prune' || true; } | crontab -
   args:
     executable: /bin/bash
   when: "'docker image prune' in root_crontab.stdout"