cmd/go: fix build config for 'go list -cover'

cmd/go: fix build config for 'go list -cover'

When 'go list -cover' is run in a way that triggers package builds
(for example, -export), ensure that the build step actually includes
coverage instrumentation as part of the config. Without this we will
wind up with incorrect build IDs.

Fixes #60755.

Change-Id: Ic84ab9e301d075bee5ff9d6828370a1708be0035
Reviewed-on: https://go-review.googlesource.com/c/go/+/502877
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>