misc/dashboard/builder: make -fail mode faster

Don't check out the repository if we're just going to fail everything.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7453048

misc/dashboard/builder/main.go

@@ -83,7 +83,7 @@ func main() {
 	}
 
 	// set up work environment, use existing enviroment if possible
-	if goroot.Exists() {
+	if goroot.Exists() || *failAll {
 		log.Print("Found old workspace, will use it")
 	} else {
 		if err := os.RemoveAll(*buildroot); err != nil {