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://golang.org/cl/147960044

dashboard/coordinator/main.go

@@ -275,7 +275,7 @@ func (conf buildConfig) dockerRunArgs(rev string) (args []string) {
 		"-rev="+rev,
 		"-dashboard="+conf.dashURL,
 		"-tool="+conf.tool,
-		"-buildroot=/"+conf.tool,
+		"-buildroot=/",
 		"-v",
 	)
 	if conf.cmd != "" {