fix(iOS): fix Prettier lint failure in headers-xcframework test

The Flow comment-type annotation on the uninitialized `let tmp` was
reformatted by Prettier (moving the comment past the semicolon and
detaching the annotation), which broke the lint job's format-check step.
Giving `tmp` an initializer keeps the `/*: string */` annotation inline
and satisfies Prettier.

Author: cipolleschi

packages/react-native/scripts/ios-prebuild/__tests__/headers-xcframework-test.js

@@ -20,7 +20,7 @@ const os = require('os');
 const path = require('path');
 
 describe('buildDepsHeadersXcframework set-equality gate', () => {
-  let tmp /*: string */;
+  let tmp /*: string */ = '';
   beforeEach(() => {
     tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'deps-headers-test-'));
   });