Report that we're using a branch provided charmcraft channel

jobs/build-charms/builder_launchpad.py

@@ -93,6 +93,7 @@ def _lp_channels(self):
         charmcraft_channel_file = Path(self.src_path) / ".charmcraft-channel"
         if charmcraft_channel_file.exists():
             channels["charmcraft"] = charmcraft_channel_file.read_text().strip()
+            self.echo(f"Using channel from {charmcraft_channel_file}: {channels['charmcraft']}")
         return channels
 
     @cached_property