cbuildbot: Don't bootstrap chromite in /tmp

For the bootstrap stage, we pull down a copy of chromite into /tmp
and patch in chromite changes there.

For systems that do not allow executing code in /tmp, this will fail.

BUG=chromium:435339
TEST=mobbuild & trybot runs.

Change-Id: I22106eb61a28b129712ac02a362b684c9bc14e2f
Reviewed-on: https://chromium-review.googlesource.com/233554
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
1 file changed