Clear AWS_CREDENTIAL_FILE env variable

This mimics the following logic in download_from_google_storage:
https://code.google.com/p/chromium/codesearch#chromium/tools/depot_tools/download_from_google_storage.py&q=download_from_goo&sq=package:chromium&l=66
and allows downloading from public buckets even when the developers has
a busted ~/.boto file.

BUG=433814
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/739633002