[client] Overhaul temporary directory override in run_isolated
[client] Overhaul temporary directory override in run_isolated

Include the rationale for each environment, because I'll have all forgotten this
in one week.

R=747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org

Bug: 867102
Change-Id: I5f6b5f824e7bd6225a182a6c075742c560d60060
Reviewed-on: https://chromium-review.googlesource.com/1150422
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py