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

glycerine/jeaten-go-arrayof-structof,d0f/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,webfd/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc