Rework actions in `.github/workflows/` to be structured like `setup-env`

[bkiahstroud]

e.g. .github/workflows/deploy.yaml --> deploy/action.yaml

setup-env can be used as a step to a job, but currently deploy has to be the entire job (not just a step). This means that if we need to add any custom behavior to the deploy flow, we can’t use the one from the Actions repo at all; we have to copy-paste it into the repo locally and alter it from there (example).

Restructuring these actions will allow us to use them in steps while allowing the flexibility to make project-specific adjustments.