os/exec: start checking for context cancelation in Start

Previously we started checking for context cancelation in Wait, but
that meant that when using StdoutPipe context cancelation never took
effect.

Fixes #16222.

Change-Id: I89cd26d3499a6080bf1a07718ce38d825561899e
Reviewed-on: https://go-review.googlesource.com/24650


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>