cmd/buildlet/stage0: support building with Go 1.8 again

While debugging a potential Go 1.9 windows networking crash, revert
the use of Go 1.9's Time.Duration so we can still build for Go 1.8.

Updates golang/go#21153

Change-Id: I4845910cd0ef376d4891a1802b0c9bcb8f7c5a0a
Reviewed-on: https://go-review.googlesource.com/51171
Reviewed-by: Sarah Adams <[email protected]>

Author: bradfitz

cmd/buildlet/stage0/stage0.go

@@ -184,7 +184,8 @@ func awaitNetwork() bool {
 		}
 		failAfter := time.Since(t0)
 		if now := time.Now(); now.After(lastSpam.Add(5 * time.Second)) {
-			log.Printf("network still down; probe failure took %v", failAfter.Round(time.Second/10))
+			const round = time.Second / 10
+			log.Printf("network still down; probe failure took %v", failAfter/round*round)
 			lastSpam = now
 		}
 		time.Sleep(1 * time.Second)