kopia: Test failure: Timeouts in blob/s3 tests

Manifests as:

2020/02/20 02:39:41 got error unable to determine if bucket "kopia-test-618dbaf8f3ffe4e6" exists:
  Access Denied. when New() S3 storage (#0), sleeping for 1s before retrying

All the retries fail, and eventually the test times out.

FAIL github.com/kopia/kopia/repo/blob/s3 90.041s