ci: keep going when the Docker image prune fails

.github/workflows/quality.yml

@@ -49,7 +49,8 @@ jobs:
               /opt/microsoft \
               /usr/local/julia* \
               /usr/local/share/chromium
-            sudo docker image prune --all --force > /dev/null
+            sudo docker image prune --all --force > /dev/null ||
+              echo "::warning::Could not remove the preinstalled Docker images."
             df -h /
           fi
       - name: Install native build dependencies