fetch-downstream: Log when the cached data is used

Ensure a log message is printed when loading the cache to give a clearer
context when debugging issues.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

bin/packaging-data-fetch-downstream

@@ -399,6 +399,7 @@ if __name__ == "__main__":
     logging.basicConfig(level=args.loglevel or logging.INFO)
 
     if args.cache:
+        logging.info(f"Loading cacheable data from {args.cache.name}")
         yaml_import = yaml.safe_load(args.cache)
         try:
             cache = yaml_import["packages"]