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?

make test ターゲットと issue で言及されている x-compile パスから始め、書き込み不可の Go stdlib ディレクトリで失敗を再現します。これらのターゲットが go install をどのように呼び出すかを追跡し、その環境で両方の操作が権限エラーなしに完了することを完了条件とします。

索引モデルが issue の本文から書いたものです。