format-disks used to run with non-strict bash semantics, but this changed in
1.2 as we now merge it into the GCE script, so pipefail and errexit are both
set.

However, the way we list the ephemeral disks, by piping to grep, would cause an
exit code of 2 if there were no ephemeral disks.

Tolerate failure here by add `|| true`.  The metadata service call is unlikely
to fail, so we continue to ignore that possibility.