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

cmd/nm: use the test binary as 'nm' 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: Iffdc60d444a9ff1d4ff5e688bca1c2ef0dfa03c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/450703
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>