cmd/go: fix broken mod_outside test
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 <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,golang/go,golang/go,cloudflare/go,nawawi/go