message: tensorflow/go: define GOCACHE
  Newer versions of go fail with:
  + go test github.com/tensorflow/tensorflow/tensorflow/go
  github.com/tensorflow/tensorflow/tensorflow/go/op build cache is
  required, but could not be located: GOCACHE is not defined and neither
  $XDG_CACHE_HOME nor $HOME are defined
diff: @@ -40,6 +40,7 @@ fi
  # Setup a GOPATH that includes just the TensorFlow Go API.
  export GOPATH="${TEST_TMPDIR}/go"
  +export GOCACHE="${TEST_TMPDIR}/cache"
  mkdir -p "${GOPATH}/src/github.com/tensorflow"
  ln -s "${PWD}" "${GOPATH}/src/github.com/tensorflow/tensorflow"
  