cmd/go: disable implicit $GOCACHE when clearing $HOME in test

Change-Id: Ie9967c8aaf7cf2e90a442937df21b1218f7ae26f
Reviewed-on: https://go-review.googlesource.com/75472
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/go_test.go

@@ -155,6 +155,9 @@ func TestMain(m *testing.M) {
 		os.Setenv("CCACHE_DIR", filepath.Join(home, ".ccache"))
 	}
 	os.Setenv("HOME", "/test-go-home-does-not-exist")
+	if os.Getenv("GOCACHE") == "" {
+		os.Setenv("GOCACHE", "off") // because $HOME is gone
+	}
 
 	r := m.Run()