build: need to specify tag for external workflow (#11224)

Author: chingor13

.github/workflows/codegen.yaml

@@ -7,7 +7,7 @@ on:
 name: codegen
 jobs:
   discovery:
-    uses: googleapis/discovery-artifact-manager/.github/workflows/list-services.yml
+    uses: googleapis/discovery-artifact-manager/.github/workflows/list-services.yml@master
 
   generate:
     runs-on: ubuntu-latest