The `TaskUpdateRequest` struct we send to task runner update hooks was not
populating the Nomad token that we get from the task runner (which we do for the
Vault token). This results in task runner hooks like the template hook
overwriting the Nomad token with the zero value for the token. This causes
in-place updates of a task to break templates (but not other uses that rely on
identity but don't currently bother to update it, like the identity hook).
