make test (and x-compile) fails if stdlib folder is not writeable

Hi,

I've given `hellogopher` a try for an internal project at work to replace our own homegrown Makefile but I'm stumped on this error:

```
$ make test
go install errors: open /usr/local/go/pkg/darwin_amd64_race/errors.a: permission denied
go install internal/race: open /usr/local/go/pkg/darwin_amd64_race/internal/race.a: permission denied
go install encoding: open /usr/local/go/pkg/darwin_amd64_race/encoding.a: permission denied
go install unicode/utf16: open /usr/local/go/pkg/darwin_amd64_race/unicode/utf16.a: permission denied
go install unicode/utf8: open /usr/local/go/pkg/darwin_amd64_race/unicode/utf8.a: permission denied
go install crypto/subtle: open /usr/local/go/pkg/darwin_amd64_race/crypto/subtle.a: permission denied
go install crypto/internal/cipherhw: open /usr/local/go/pkg/darwin_amd64_race/crypto/internal/cipherhw.a: permission denied
go install internal/nettrace: open /usr/local/go/pkg/darwin_amd64_race/internal/nettrace.a: permission denied
go install container/list: open /usr/local/go/pkg/darwin_amd64_race/container/list.a: permission denied
go install vendor/golang_org/x/crypto/curve25519: open /usr/local/go/pkg/darwin_amd64_race/vendor/golang_org/x/crypto/curve25519.a: permission denied
go install unicode: open /usr/local/go/pkg/darwin_amd64_race/unicode.a: permission denied
go install math: open /usr/local/go/pkg/darwin_amd64_race/math.a: permission denied
make: *** [test] Error 1
```

I'm running:
```
$ go version
go version go1.8rc2 darwin/amd64
```

Any tips on where to look to try to get this working?

Comienza con el target make test y la ruta x-compile mencionada en el issue, y luego reproduce el fallo con un directorio de Go stdlib no escribible. Rastrea cómo estos targets invocan go install y define como completado que ambas operaciones finalicen sin errores de permisos en ese entorno.

Escrito por el modelo de indexación a partir del texto del issue.