fix: expose missing audiences variable for gitlab workflow file generation (#3385)

In 0-org-setup, when using gitlab cicd, workflow-gitlab.yaml expects an
audiences variable.

Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>

fast/stages/0-org-setup/cicd.tf

@@ -42,6 +42,7 @@ locals {
           v.workload_identity_provider.id,
           v.workload_identity_provider.id
         )
+        audiences = try(v.workload_identity_provider.audiences, [])
         service_accounts = {
           apply = lookup(
             local.of_service_accounts,