isolatedclient: use ctx for context.Context
isolatedclient: use ctx for context.Context

Derived from
https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2133333/3/common/isolatedclient/isolatedclient.go#320

Bug: 1067133
Change-Id: Ie0b216ae1f3ff9c8f74c9953dc2d248db14a4a87
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2133334
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@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