x/tools/dashboard/coordinator: provide build key in / and /root
x/tools/dashboard/coordinator: provide build key in / and /root

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/170260044

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