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

cmd/pack: use the test binary as 'pack' 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: Ifaf396465f54ad0a28977b6c0a4b87dbc359de37
Reviewed-on: https://go-review.googlesource.com/c/go/+/450706
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>