Firebase Next.js webframeworks rewrites to external destinations do not work

### [REQUIRED] Environment info

The one from: https://github.com/FirebaseExtended/action-hosting-deploy/tree/v0/ as I'm deploying with Github Actions. I guess it's the latest.

**Platform:** Linux

### [REQUIRED] Test case

1. See the following public repo: https://github.com/gusarov-studio/langfast/tree/boilerplate
2. I've followed all of the steps from the official guideline: https://firebase.google.com/docs/hosting/frameworks/nextjs
3. I've configured a simple rewrite rule to external destination in [next.config.ts](https://github.com/gusarov-studio/langfast/blob/54f6d84c8303ef77ce6a9cfbee5173b376d76b0e/hosting/next.config.ts#L27-L44)
4. All requests to any external destination rewrites fail with 504 Gateway Timeout

### [REQUIRED] Steps to reproduce
I've depoyed the app temporarily for you:
1. Simply go to: https://langfast--pr1-boilerplate-akkcei85.web.app/studio or https://langfast--pr1-boilerplate-akkcei85.web.app/replay-xF5c/script.js

### [REQUIRED] Expected behavior

The target external destinations are successfully proxied through the Cloud Function that Firebase creates automatically.

### [REQUIRED] Actual behavior

504 Gateway Timeout error