unix: fix TestSCMCredentials to not fail when root
unix: fix TestSCMCredentials to not fail when root

Fixes golang/go#25354

Change-Id: Ic74005ed26792a78354ffe76f567e9a009eabe9a
Reviewed-on: https://go-review.googlesource.com/112738
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys