cmd/nm: don't rely on an erroneous install target in tests

cmd/nm: don't rely on an erroneous install target in tests

Non-main packages in module mode should not be installed to
GOPATH/pkg, but due to #37015 they were installed there anyway.
This change switches the 'go install' command in testGoLib to instead
use 'go build -buildmode=archive' with an explicit output file.

For #37015.

Change-Id: I15781aa33d1b2adc6a4437a58622276f4e20b889
Reviewed-on: https://go-review.googlesource.com/c/go/+/416955
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>