This change fixes a bug where nomad would avoid running alloc tasks if
the alloc is client terminal but the server copy on the client isn't
marked as running.

Here, we fix the case by having task runner uses the
allocRunner.shouldRun() instead of only checking the server updated
alloc.

Here, we preserve much of the invariants such that `tr.Run()` is always
run, and don't change the overall alloc runner and task runner
lifecycles.

Fixes #5883