cmd/vet: use the test binary as 'vet' instead of rebuilding it

cmd/vet: use the test binary as 'vet' instead of rebuilding it

This not only reduces the latency of the test, but also respects
build flags like '-race' and '-cover' passed to the 'go test' command.

Change-Id: I4914870396f90097f45d00012e5601fe42655899
Reviewed-on: https://go-review.googlesource.com/c/go/+/450710
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>