Store the RNTester artifacts to speed-up E2E (#48442)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48442

This change stores the RNTester `.app` in an artifact so that E2E tests can reuse it.

## Context

While looking at the recent failures of the E2E tests, I realized that the Hermes, NewArch, Debug variant often fails to build, not to test, for some misconfiguration.

I also realized that we are already building that varaint successfully once, so why not reuse it? To reuse prebuilds, we need a few steps:

1. make sure we build all the variants we need
2. store the .app file as an artifact
3. download the artifact and use it in the E2E tests

## Changelog:
[Internal] - Build release variant for RNTester

Reviewed By: mdvacca

Differential Revision: D67760380

fbshipit-source-id: 8be0bbbadf4240dce1bcf5b44dadb41d49ed4c06