x/tools/dashboard/app: ignore freebsd-arm failures
x/tools/dashboard/app: ignore freebsd-arm failures

The previous instance of this change was applied to the release branch.
I don't know why the codereview plugin allowed me to do this.
Thankfully we won't be using it for much longer.

TBR=dfc
R=dave
CC=golang-codereviews
https://codereview.appspot.com/175870044

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