cmd/go: fix syntax mistake in a testscript file, take 2

This is a followup to CL 181278 and CL 181177.

According to cmd/go/testdata/script/README:

	Each line is parsed into a sequence of space-separated command words,
	with environment variable expansion and # marking an end-of-line comment.
	Adding single quotes around text keeps spaces in that text from being treated
	as word separators and also disables environment variable expansion.

We want $HOME to be expanded, so leave it out of the single-quoted
block of text.

I tested this change on macOS, and it makes TestScript/env_write pass.

Fixes #32503

Change-Id: I13621aec82263e5cb6978c13a1ad71d2210a0e42
Reviewed-on: https://go-review.googlesource.com/c/go/+/181418


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>