cache: fix test not to leave an empty dir
cache: fix test not to leave an empty dir

This test left an empty dir near the cwd.
So changed to use temporal dir as cwd instead.

Change-Id: Ie9174c5b534048d3c311200d80dc1867becd9469
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2968905
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go