fix: use unstable Docker image ID in test-client workflow instead of latest when triggered by the nightly dispatcher

Author: dlachaume

.github/workflows/test-client.yml

@@ -46,7 +46,7 @@ on:
         type: string
       docker_image_id:
         type: string
-        default: latest
+        default: unstable
       network:
         type: string
         default: preview