Fix analyze_code to use the proper PR number (#46548)

Summary:
Analyze code was still using the CIRCLECI env variable to retrieve the PR number.
This change uses the github one and also removes a duplicated flow check

## Changelog:
[Internal] - Remove duplicated flow check and use gh PR number rather than CircleCI one

Pull Request resolved: https://github.com/facebook/react-native/pull/46548

Test Plan: Tested locally and in CI

Reviewed By: NickGerleman

Differential Revision: D62883758

Pulled By: cipolleschi

fbshipit-source-id: 0c7e97ea9caf4465972424e0b69a899ecfd07095