go.tools/dashboard/builder: fix imported by not used error

Update golang/go#7372

This slipped into the build in revision 4b4229fc616b.

Thanks to @pierredurand87 for noticing the break.

LGTM=bgarcia
R=golang-codereviews, bgarcia
CC=golang-codereviews
https://golang.org/cl/66670043

builder/env.go

@@ -7,7 +7,6 @@ package main
 import (
 	"bytes"
 	"fmt"
-	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"