task runner: fix goroutine leak in prestart hook

The task runner prestart hooks take a `joincontext` so they have the
option to exit early if either of two contexts are canceled: from
killing the task or client shutdown. Batch jobs (and any failed job)
exit without being shutdown from the server, so neither of the joined
contexts ever gets canceled and we leak the `joincontext` (48 bytes)
and its internal goroutine. Cancel the `joincontext` after the
prestart call exits to fix the leak.

.changelog/11741.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: Fixed a memory and goroutine leak for batch tasks and any task that exits without being shut down from the server
+```

client/allocrunner/taskrunner/task_runner_hooks.go

@@ -237,7 +237,9 @@ func (tr *TaskRunner) prestart() error {
 		// Run the prestart hook
 		// use a joint context to allow any blocking pre-start hooks
 		// to be canceled by either killCtx or shutdownCtx
-		joinedCtx, _ := joincontext.Join(tr.killCtx, tr.shutdownCtx)
+		joinedCtx, joinedCancel := joincontext.Join(tr.killCtx, tr.shutdownCtx)
+		defer joinedCancel()
+
 		var resp interfaces.TaskPrestartResponse
 		if err := pre.Prestart(joinedCtx, &req, &resp); err != nil {
 			tr.emitHookError(err, name)