dashboard/coordinator: restore buildroot to /
dashboard/coordinator: restore buildroot to /

So the optimization to avoid hg clone works again.  The
builder looks at buildroot+"goroot" for an hg clone repo. If
it finds it, it can just "hg pull" instead of a slow "hg
clone".

LGTM=cmang
R=cmang, proppy
CC=golang-codereviews
https://codereview.appspot.com/147960044

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools