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>

src/cmd/go/testdata/script/env_write.txt

@@ -5,7 +5,7 @@ env AppData=$HOME/windowsappdata
 env home=$HOME/plan9home
 go env GOENV
 [aix] stdout $HOME/.config/go/env
-[darwin] stdout $HOME/Library/Application Support/go/env
+[darwin] stdout '$HOME/Library/Application Support/go/env'
 [freebsd] stdout $HOME/.config/go/env
 [linux] stdout $HOME/.config/go/env
 [netbsd] stdout $HOME/.config/go/env