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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go