Add desktop workflows to doctor (#31283)

The 'doctor' check to determine if any devices can be listed for the
device should consider desktop workflows as well.

This has no effect unless the environment variable to enable desktop
device listing is set.

packages/flutter_tools/lib/src/doctor.dart

@@ -25,10 +25,13 @@ import 'globals.dart';
 import 'intellij/intellij.dart';
 import 'ios/ios_workflow.dart';
 import 'ios/plist_utils.dart';
+import 'linux/linux_workflow.dart';
+import 'macos/macos_workflow.dart';
 import 'proxy_validator.dart';
 import 'tester/flutter_tester.dart';
 import 'version.dart';
 import 'vscode/vscode_validator.dart';
+import 'windows/windows_workflow.dart';
 
 Doctor get doctor => context[Doctor];
 
@@ -89,6 +92,16 @@ class _DefaultDoctorValidatorsProvider implements DoctorValidatorsProvider {
 
       if (fuchsiaWorkflow.appliesToHostPlatform)
         _workflows.add(fuchsiaWorkflow);
+
+      if (linuxWorkflow.appliesToHostPlatform)
+        _workflows.add(linuxWorkflow);
+
+      if (macOSWorkflow.appliesToHostPlatform)
+        _workflows.add(macOSWorkflow);
+
+      if (windowsWorkflow.appliesToHostPlatform)
+        _workflows.add(windowsWorkflow);
+
     }
     return _workflows;
   }