Playground block: Read stderr information from /internal/stderr, not /tmp/stderr

Playground compatibility update after merging WordPress/wordpress-playground#1063

Author: adamziel

packages/wordpress-playground-block/src/components/playground-preview/index.tsx

@@ -103,7 +103,7 @@ export default function PlaygroundPreview({
 		withErrorLog: codeEditorErrorLog,
 		getErrors: async () =>
 			(await playgroundClientRef.current?.readFileAsText(
-				'/tmp/stderr'
+				'/internal/stderr'
 			)) || '',
 	});