cmd/go: fix broken mod_outside test

Since CL 254369, 'go mod graph' now reports an error when invoked
outside a module. This broke the mod_outside test, which expected
'go mod graph' to succeed with no output.

Change-Id: Ic30ee68f1f4c4d33795bdf7df70a7631fb9395e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/255017


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>