__fetch_polyfill not found in @sveltejs/kit/dist/install-fetch.js

```
Build failed with 1 error:
.svelte-kit/appengine-tmp/entry.js:3:8: error: No matching export in "node_modules/@sveltejs/kit/dist/install-fetch.js" for import "__fetch_polyfill"
```

Looks like the `__fetch_polyfill` is now available as `installFetch` as noted here: https://github.com/sveltejs/kit/pull/4111