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The packaging script previously would generate environment variables
with lots of ..\.. entries in environment variables, which is difficult
to read and increases the odds of hitting MAX_PATH limits.
crrev.com/c/2372727 changes the packaging and this change to
setup_toolchain.py adjusts the setup code to handle both the old and new
systems.

Bug: 1120785
Change-Id: I7c3449a54cf6fba9f70f52db83d940ffd4c32573
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370604
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801277}