os/exec: only use cachedLookExtensions if Cmd.Path is unmodified
Caching the invocation of lookExtensions on an absolute path in Command
and reusing the cached result in Start is only viable if Cmd.Path isn't
set to a different value after Command returns.
For #66586.
Fixes #68314.
Change-Id: I57007850aca2011b11344180c00faded737617b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/596875 Reviewed-by: qiu laidongfeng2 <2645477756@qq.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>