x/build: tip.golang.org is down

As of 2018-02-03 18:10:00 UTC, [tip.golang.org](https://tip.golang.org/) returns a 500 with the following body:

```
builder.Init: Building Go cmd/dist using /usr/local/go.
go build _/build/tip/b/go/src/cmd/dist: /usr/local/go/pkg/tool/linux_amd64/compile: fork/exec /usr/local/go/pkg/tool/linux_amd64/compile: resource temporarily unavailable

exit status 1
```