Explicitely set REACT_NATIVE_DOWNLOADS_DIR to use predownloaded deps (#55567)

Summary:
This updates the CI to use `REACT_NATIVE_DOWNLOADS_DIR` so the directory where the C++ dependencies are consumed from is always the same.

This works in conjuction with:
- https://github.com/react-native-community/docker-android/pull/248

## Changelog:

[INTERNAL] -

Pull Request resolved: https://github.com/facebook/react-native/pull/55567

Test Plan: CI

Reviewed By: cipolleschi

Differential Revision: D93417423

Pulled By: cortinico

fbshipit-source-id: 75664e7d48cbba2483e05c023ac0201fb768ef35