Fix idempotency in apt module (#38858)

* Idempotency fix

Only show a change, if we did indeed update the cache.

* Apply changes from review feedback

lib/ansible/modules/packaging/os/apt.py

@@ -1019,8 +1019,10 @@ def main():
                 else:
                     module.fail_json(msg='Failed to update apt cache: %s' % err)
                 cache.open(progress=None)
-                updated_cache = True
-                mtimestamp, updated_cache_time = get_updated_cache_time()
+                mtimestamp, post_cache_update_time = get_updated_cache_time()
+                if updated_cache_time != post_cache_update_time:
+                    updated_cache = True
+                updated_cache_time = post_cache_update_time
 
             # If there is nothing else to do exit. This will set state as
             #  changed based on if the cache was updated.