fix(preview): add user builderPlugin to iframe builder (#838)

Author: 10Derozan

.changeset/strange-months-glow.md

@@ -0,0 +1,5 @@
+---
+'@rspress/plugin-preview': patch
+---
+
+fix: add user builderPlugin to iframe builder

packages/plugin-preview/src/index.ts

@@ -123,6 +123,7 @@ export function pluginPreview(options?: Options): RspressPlugin {
             // not copy files again
             copy: undefined,
           },
+          plugins: config?.builderPlugins,
         },
         builderConfig,
       );