Simplify oso ci-default workflow for external PRs

[ryscheng]

What is it?

Currently we do this thing where we build docker containers that embed our secrets so that we can approve and trigger external PRs that use our secrets.

There's a simpler approach here:
https://dvc.ai/blog/testing-external-contributions-using-github-actions-secrets

FWIW, we will want to continue use the GitHub app for these purposes:

• test-deploy in OSO because it runs up our GCP bill to do the dbt run. We want to trigger each run against a git commit SHA
• oss-directory validation checks, which will check for different validation criteria and have a nice way to report errors to the user's PR