Disable concurrency on main (#46435)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46435

On main, we want to run CI as fast as possible to catch failures as soon as possible.
Currently, if two commits arrives on main (say commit A and commi B) CI will run on commit B only when it is finished on commit A.
Instead, we would like for CI to run in parallel.

## Changelog
[Internal] - Run CI in parallel on main

Reviewed By: cortinico

Differential Revision: D62496842

fbshipit-source-id: 03dc910deab42af98d92ea8e8e5b90049940dba1