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. Some tasks 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. This primarily impacts batch jobs and any task that fails
or completes early such as non-sidecar prestart lifecycle tasks.
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)