Node.js 20 deprecation warning in reusable build workflow

### Description

When using the reusable workflow from `esphome/workflows/.github/workflows/build.yml@main`, GitHub Actions reports the following deprecation warning:

```
Node.js 20 actions are deprecated. The following actions are running on Node.js 20
and may not work as expected:
- actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02
- docker/build-push-action@v6.18.0
- docker/setup-buildx-action@v3.11.1
```

### Impact

GitHub has announced the following deadlines:
- **June 2, 2026** — Node.js 24 becomes the default; workflows using Node.js 20 actions may break
- **September 16, 2026** — Node.js 20 is removed from runners entirely

Since these actions are pinned inside the reusable workflow, end users cannot update them themselves.