cmd/go: don't put mycmd in wrong directory

Change-Id: Ia7cc1d52211b32a2eb2b3888d621b28d6932aca9
Reviewed-on: https://go-review.googlesource.com/12290Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -751,6 +751,7 @@ func TestGoInstallErrorOnCrossCompileToBin(t *testing.T) {
 	tg.tempFile("src/mycmd/x.go", `package main
 		func main() {}`)
 	tg.setenv("GOPATH", tg.path("."))
+	tg.cd(tg.path("src/mycmd"))
 
 	tg.run("build", "mycmd")