misc/dashboard/builder: use c:\ as default buildroot on windows
misc/dashboard/builder: use c:\ as default buildroot on windows

We have some tests (misc/cgo/test) that are disabled only because
they will fail to run on go builder - see issue 3358 for details.
This change will allow us to enable these tests.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6493118

firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old