cmd/go: fix TestIssue11457

cmd/go: fix TestIssue11457

The goal of the test is to provoke a custom import path check error,
which will contain the current repo path, to see that it says ssh:// in it.

But the fix to #16471 made the test no longer provoke that error.
Provoke the error by checking out from rsc.io instead of github.com/rsc.

Fixes #17701.

Change-Id: I750ffda2ff59c2be8e111d26160997214a73fd9a
Reviewed-on: https://go-review.googlesource.com/32641
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>