[task scheduler] Set HOME variable to tmp dir when running bot_update
[task scheduler] Set HOME variable to tmp dir when running bot_update

bot_update makes modifications to the global gitconfig, which results in
problems when running concurrently. HOME informs git of where to look
for the global gitconfig, so this should prevent conflicts.

Bug: chromium:1005388
Change-Id: If1760cb95a2c4ee687c75de541672a64d6d3367b
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/242797
Reviewed-by: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Reviewed-by: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot