Replace all third-party actions with in-house actions or reusable workflows.

We use a handful of third-party actions in our shared and repo-specific workflows. Some of them require us to pass along keys, tokens, permissions, etc. Also, we become dependent on them not releasing a breaking change to the version we've pinned to.

Ideally, we would write our own actions and workflows. Some may even be replaced with simple shell scripts.

We should:
1. Catalog the actions we use.
2. Identify how vulnerable each makes us.
3. Estimate how simple it would be to replace them.
4. Begin replacing them starting with the highest risk and easiest to replace.