template: restore driver handle on update (#15915)

When the template hook Update() method is called it may recreate the
template manager if the Nomad or Vault token has been updated.

This caused the new template manager did not have a driver handler
because this was only being set on the Poststart hook, which is not
called for inplace updates.

.changelog/15915.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+template: Fixed a bug that caused the chage script to fail to run
+```

client/allocrunner/taskrunner/template_hook.go

@@ -54,6 +54,12 @@ type templateHook struct {
 	templateManager *template.TaskTemplateManager
 	managerLock     sync.Mutex
 
+	// driverHandle is the task driver executor used by the template manager to
+	// run scripts when the template change mode is set to script.
+	//
+	// Must obtain a managerLock before changing. It may be nil.
+	driverHandle ti.ScriptExecutor
+
 	// consulNamespace is the current Consul namespace
 	consulNamespace string
 
@@ -124,7 +130,8 @@ func (h *templateHook) Poststart(ctx context.Context, req *interfaces.TaskPostst
 	}
 
 	if req.DriverExec != nil {
-		h.templateManager.SetDriverHandle(req.DriverExec)
+		h.driverHandle = req.DriverExec
+		h.templateManager.SetDriverHandle(h.driverHandle)
 	} else {
 		for _, tmpl := range h.config.templates {
 			if tmpl.ChangeMode == structs.TemplateChangeModeScript {
@@ -158,6 +165,9 @@ func (h *templateHook) newManager() (unblock chan struct{}, err error) {
 	}
 
 	h.templateManager = m
+	if h.driverHandle != nil {
+		h.templateManager.SetDriverHandle(h.driverHandle)
+	}
 	return unblock, nil
 }