Fix checkout retries mechanism.

The checkouts were actually failing because the [cache]/git was in a bad
state. This CL removes that cache before retrying and ensure vpython is
avaialable.

Change-Id: I3b5ef5e3fc1345f44c740eebc72144313dfce8ea
Bug: https://github.com/flutter/flutter/issues/63100
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5460
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>