os/exec: avoid calling LookPath in cmd.Start for resolved paths
This reapplies CL 512155, which was previously reverted in CL 527337.
The race that prompted the revert should be fixed by CL 527820,
which will be submitted before this one.
For #36768.
Updates #62596.
Change-Id: I3c3cd92470254072901b6ef91c0ac52c8071e0a2
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-race,gotip-windows-amd64-race,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/528038
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>