cmd/go: do not exit with non-zero code from go list -e -export

cmd/go: do not exit with non-zero code from go list -e -export

go list -e -export puts errors running build actions on the load.Package
corresponding to the failed action rather than exiting with a non zero
exit code.

For #25842

Change-Id: I1fea85cc5a0557f514fe9d4ed3b6a858376fdcde
Reviewed-on: https://go-review.googlesource.com/c/go/+/437298
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>