Add context to net-lb-app-ext module (#3879)

modules/net-lb-app-ext/backends.tf

@@ -20,7 +20,7 @@ resource "google_compute_backend_bucket" "default" {
   for_each = var.backend_buckets_config
   project = (
     each.value.project_id == null
-    ? var.project_id
+    ? local.project_id
     : each.value.project_id
   )
   name                    = coalesce(each.value.name, "${var.name}-${each.key}")