use ${vpython_platform} for coverage in .vpython3
use ${vpython_platform} for coverage in .vpython3

Fixed: 1100211
Change-Id: I3d23a9c4be720edd2fcca633f2c58fc100dae595
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3013131
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>

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