Fix checkout retry code.

We need to split the checkout code into two pieces: the one that runs on
a cwd context and the one deleting the folder used in the context.

Change-Id: I3ad8c4b9daa7fe0754aad65fd344fa00346a71f3
Bug: https://github.com/flutter/flutter/issues/59337
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/3600
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>