Update tnull Go/Starlark to work better

* Use current proto messages to get compilation working.
* Create local loop vars in driver.go, since those weren't
  going to work before. e.g. https://play.golang.org/p/ll6GwC3dMVA
* Make a simple update to generate_metadata.sh to switch to correct
  directory in case you start elsewhere.

TEST=emerge tnull; ./generate_metadata.sh

Change-Id: I963046ad8b61f9a4be4e7e9f6538562a7eeba98b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/tnull/+/2472081
Tested-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@google.com>
Commit-Queue: Sean Abraham <seanabraham@chromium.org>
3 files changed