cmd/go: 'go build' error message says 'go install'

At tip as of May 4 (8d63408f4688ff577c25f07a1728fe131d0cae2a):

```bash
$ go build zombiezen.com/go/capnproto2/capnpc-go
go install zombiezen.com/go/capnproto2/capnpc-go: build output "capnpc-go" already exists and is a directory
```

The error message should begin "go build ...".
