x/tools/dashboard/app: prevent builders from nuking other results
x/tools/dashboard/app: prevent builders from nuking other results

This fixes the issue where a builder would occasionally wipe out
the results from another builder.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/172150043

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