misc/dashboard/builder: set GOPATH before building subrepos

This proposal updates the dashboard builder to avoid relying on the (soon to be removed) support for using go get to download to $GOROOT. The result is

WORKSPACE=$(the value of the -buildRoot flag / $BUILDER_NAME + hg revision)
GOROOT=$WORKSPACE/go
GOPATH=$WORKSPACE

Required for CL 6941058.

R=minux.ma, adg
CC=golang-dev
https://codereview.appspot.com/7034049