Support AMPLIFY_MONOREPO_APP_ROOT environment variable on branch level

### Before opening, please confirm:

- [x] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [x] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Monorepo

### Is your feature request related to a problem? Please describe:

The AMPLIFY_MONOREPO_APP_ROOT environment variable can only be set at the app-level and not at the branch-level.

We are working on improving our monorepo setup on a different branch within our Amplfiy app, but this is currently not possible with Amplify hosting. This is not clear from the documentation.

I have verified this with AWS Support, creating this feature request for tracking.

### Describe how you'd like this feature to work

Usage of AMPLIFY_MONOREPO_APP_ROOT should be possible to use from branch level within an Amplfiy app.

Start by reviewing the Amplify Hosting monorepo feature and how AMPLIFY_MONOREPO_APP_ROOT is currently configured at app level. Done means the environment variable can be configured independently for each branch, with the behavior and documentation making that scope clear.

Written by the indexing model from the issue text.