client: ensure task only runs with prestart hooks (#18662)

Since the allocation in the task runner is updated in a separate
goroutine, a race condition may happen where the task is started but the
prestart hooks are skipped because the allocation became terminal.

Checking for a terminal allocation before proceeding with the task start
ensures the task only runs if the prestart hooks are also executed.

Since `shouldShutdown()` only uses terminal allocation status, it
remains `true` after the first transition, so it's safe to check it
again after the prestart hooks as it will never revert to `false`.

.changelog/18662.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: prevent tasks from starting without the prestart hooks running
+```

client/allocrunner/taskrunner/task_runner.go

@@ -599,6 +599,12 @@ MAIN:
 			goto RESTART
 		}
 
+		// Check for a terminal allocation once more before proceeding as the
+		// prestart hooks may have been skipped.
+		if tr.shouldShutdown() {
+			break MAIN
+		}
+
 		select {
 		case <-tr.killCtx.Done():
 			break MAIN