fixes [`Scrollbar.thickness` property is ignored when the `Scrollbar` is hovered](https://github.com/flutter/flutter/issues/143881)

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    final ScrollController scrollController = ScrollController();

    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Material(
        child: ScrollConfiguration(
          behavior: const NoScrollbarBehavior(),
          child: ScrollbarTheme(
            data: ScrollbarThemeData(
              thickness: MaterialStateProperty.all(25.0),
              showTrackOnHover: true,
            ),
            child: Scrollbar(
              thickness: 50.0,
              thumbVisibility: true,
              radius: const Radius.circular(3.0),
              controller: scrollController,
              child: SingleChildScrollView(
                controller: scrollController,
                child: const SizedBox(width: 4000.0, height: 4000.0),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class NoScrollbarBehavior extends ScrollBehavior {
  const NoScrollbarBehavior();

  @override
  Widget buildScrollbar(
          BuildContext context, Widget child, ScrollableDetails details) =>
      child;
}
```

</details>

### Preview

| Before | After |
| --------------- | --------------- |
| <img src="https://github.com/flutter/flutter/assets/48603081/3537d60d-a5b2-488d-aa99-4c36c3721657"  /> | <img src="https://github.com/flutter/flutter/assets/48603081/cfd02095-a327-4b16-8ece-0d1c9e6813ce" /> |