Ctrl-Cing out of galera_new_cluster doesn't reset systemd environment

Excerpt from /bin/galera_new_cluster:

systemctl set-environment _WSREP_NEW_CLUSTER='--wsrep-new-cluster' && \

    systemctl start ${1:-mariadb}

systemctl set-environment _WSREP_NEW_CLUSTER=''

If the service start takes too long and the user aborts the script, the environment never gets reset and subsequent attempt to join this node to another primary component via `systemctl start mariadb` will instead bootstrap it.