[client] import subprocess in local_caching.py
[client] import subprocess in local_caching.py

subprocess isn't imported.
https://chromeos-swarming.appspot.com/task?id=4d2073cbc6c97910&o=true

Change-Id: Ic39db29cfdff1e798ec0c2702e2496908381301c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2277363
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Deepa Kurian <2358e686997cf5aebaa47a0357e062f4348bf58b@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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