On windows on Python 2, only strings can be environment variables, but
literals are unicode, so we need to convert to strings before setting
env vars.

Change-Id: I40e8e6ae4f5497d56efa568153eb65392dbd85e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1875108Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>