As reported in crbug.com/1122671, windows bots can manage to trigger
the isatty conditional even when openpty() isn't provided.

So just force that conditional to false on windows.

Bug: 1122671
Change-Id: I2c85024da40feadd97cde7f718f4e5a4231fe9d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2386321Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>