When a node becomes ready, create an eval for all system jobs across
namespaces.

The previous code uses `job.ID` to deduplicate evals, but that ignores
the job namespace. Thus if there are multiple jobs in different
namespaces sharing the same ID/Name, only one will be considered for
running in the new node. Thus, Nomad may skip running some system jobs
in that node.

NamespacedID is never nil and is very light (two string fields) that
isn't in the hot path. So it's safe to return and use the struct
directly.

Avoiding using pointers here avoids the risk of dereferencing a nil
pointer.