Add packed `.tgz` as build artifact to aid in testing PRs

Testing PRs against `test-site` is pretty straightforward, however, the current "test someone's PR against an app" method is:
* Pull down PR branch
* Pack `frontend-base`
* Install newly packed `frontend-base` in the app you're testing it against

It might be cool to be able to grab a `.tgz` from a PR to save a couple steps and just:
* Download `.tgz` from CI run
* Install/test against app
