Explicitly show "Currently not on an official channel." (flutter#84992)

Author: asashour

packages/flutter_tools/lib/src/commands/channel.dart

@@ -113,6 +113,11 @@ class ChannelCommand extends FlutterCommand {
         }
       }
     }
+
+    if (currentChannel == 'unknown') {
+      globals.printStatus('');
+      globals.printStatus('Currently not on an official channel.');
+    }
   }
 
   Future<void> _switchChannel(String branchName) async {

packages/flutter_tools/test/general.shard/channel_test.dart

@@ -159,7 +159,7 @@ void main() {
         .where((String line) => line?.isNotEmpty == true)
         .skip(1); // remove `Flutter channels:` line
 
-      expect(rows, <String>['dev', 'beta', 'stable']);
+      expect(rows, <String>['dev', 'beta', 'stable', 'Currently not on an official channel.']);
     }, overrides: <Type, Generator>{
       ProcessManager: () => fakeProcessManager,
       FileSystem: () => MemoryFileSystem.test(),