prpc: use ctx for context.Context
prpc: use ctx for context.Context

This is for https://screenshot.googleplex.com/7JEGrE58mKjTebY

Bug: 1143123
Change-Id: I42db3bae82a5802df0f5b8ba3a98636f177ba2c9
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3012891
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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