cmd/go: fix syntax mistake in a testscript file

cmd/go: fix syntax mistake in a testscript file

I didn't realise that the trybots don't include any Mac machines, so I
assumed this test change was fine when submitting CL 181177.

In any case, this is a simple fix. I forgot to add the quotes, as the
new UserConfigDir on Mac includes a space.

Change-Id: I0766b966fc41736e9fc859e37f059a3f12788d7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/181278
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>