isolated: add cache support in download sub command
isolated: add cache support in download sub command

Bug: 932396
Change-Id: Ib5c7f9f457fe58373c65f24a1aa8812a7482d6d8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1872070
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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