Fix bugs in Cloud SDK install

- Store tarball in workspace
- Replace erroneous references to gcompute with gcloud, as intended
- Make a clearly buggy attempt to handle architectures, but it's
  working?!
    - We're building amd64 images, but it's managing to install Cloud
      SDK, even though I accidentally listed the x86_64 tarball under
      i386.
    - Only the Go bits of the App Engine SDK are architecture-specific.
      Do we care about architecture for the current install bundle?