Update cocoon recipe to use repo_utils module.

This has the side effect of removing the hardcoded urls that were
preventing testing changes required for github checks api.

Change-Id: Ib4b58e868127ac110f15395c4aaad91cb52793a7
Bug: https://github.com/flutter/flutter/issues/56422
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/3882
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>