cmd/go: fix TestScript/test_main_twice

The TMPDIR environment variable isn't always available. Use $WORK/tmp
instead, to fix this test on Windows.

Change-Id: I521c470430c6eea4d1f2fc7db749d38178a03a13
Reviewed-on: https://go-review.googlesource.com/c/go/+/213680
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

 [short] skip
 
-env GOCACHE=$TMPDIR
+env GOCACHE=$WORK/tmp
 go test -v multimain
 stdout -count=2 notwithstanding # check tests ran twice