Make the appengine service name sweepable in the test compute region network endpoint (#11993)

Author: zli82016

mmv1/templates/terraform/examples/region_network_endpoint_group_appengine.tf.tmpl

@@ -11,7 +11,7 @@ resource "google_compute_region_network_endpoint_group" "{{$.PrimaryResourceId}}
 
 resource "google_app_engine_flexible_app_version" "{{$.PrimaryResourceId}}" {
   version_id = "v1"
-  service    = "appengine-network-endpoint-group"
+  service    = "{{index $.Vars "neg_name"}}"
   runtime    = "nodejs"
   flexible_runtime_settings {
     operating_system = "ubuntu22"