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>

src/cmd/go/go_test.go

@@ -1216,7 +1216,7 @@ func TestIssue11457(t *testing.T) {
 	tg.parallel()
 	tg.tempDir("src")
 	tg.setenv("GOPATH", tg.path("."))
-	const importPath = "github.com/rsc/go-get-issue-11457"
+	const importPath = "rsc.io/go-get-issue-11457"
 	tg.run("get", "-d", "-u", importPath)
 	repoDir := tg.path("src/" + importPath)
 	tg.runGit(repoDir, "remote", "set-url", "origin", "git@github.com:rsc/go-get-issue-11457")