misc/dashboard/builder: add environment variables for Plan 9
misc/dashboard/builder: add environment variables for Plan 9

We require $objtype in make.rc and rc needs $path for finding commands.

Also include $cputype which we may use in the future.

R=golang-dev, minux.ma, r
CC=golang-dev
https://codereview.appspot.com/9905043

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