cmd/go: fix TestScript/test_main_twice
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 <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,akutz/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,golang/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go