[cipd] Handle SIGTERM and Ctrl+C by canceling the root context.
[cipd] Handle SIGTERM and Ctrl+C by canceling the root context.

This is needed to make sure the client properly cleans up various
temp files if it is interrupted in the middle of a download.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org
BUG=1217435

Change-Id: I7ef5d24cfbc424c453e41342de7598989937d08e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2965762
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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