This makes it easier to figure out which execution was last when looking
at the output of `systemd list-units kubelet-*.service`.

We try to find the name of the /tmp/node-e2e-* directory and use the
same timestamp if we can. Otherwise, we just call Now() again, which
isn't as nice (as the unit name and directory name will not match) but
will still produce unit names that will be ordered when launching
multiple subsequent executions on the same host.