cmd/go: stop using $GOROOT and $GOBIN in script output
cmd/go: stop using $GOROOT and $GOBIN in script output

They were necessary to produce a canonical script
when we checked in build scripts, but now they're just
getting in the way.

Fixes issue 3279.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5796068

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