build: AR exit gate YAML to declare dependencies.gitSource (#3929)

This should suppress the warning in b/450542318.

Author: suztomo

internal/librariangen/cloudbuild-exitgate.yaml

@@ -42,4 +42,11 @@ options:
     requestedVerifyOption: VERIFIED  # For provenance attestation generation
     logging: CLOUD_LOGGING_ONLY
 images:
-  - us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest
+  - us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest
+dependencies:
+- gitSource:
+    repository:
+      url: "${_LOUHI_REPOSITORY_URL}"
+    revision: "${_LOUHI_REF_SHA}"
+    destPath: "."
+