flatpak-update-worker.py: Use installer.init() if init_sync() fails,

instead of our own refresh method.

This will make sure the appstream signal is fired once the cache
updates (and matches mintinstall's behavior as well).

usr/lib/linuxmint/mintUpdate/flatpak-update-worker.py

@@ -106,7 +106,7 @@ def fetch_updates(self):
 
         if not self.installer.init_sync():
             warn("cache not valid, refreshing")
-            self.refresh(False)
+            self.installer.init()
         else:
             debug("cache valid")