[WIP] Make it work in Actions v2

[shawnbot]

This probably needs a full rewrite, but for now let's see if we can make this work for primer/css#881. All you should need in your workflow.yml is:

jobs:
  whatever:
    - uses: actions/checkout@master
    - uses: actions/setup-node@master
    - uses: primer/publish@actions-v2

[shawnbot]

I'm going to close this PR and open a new one that updates the README with instructions for using the action as-is with v2. Thanks to @colebemis for saving me a bunch of time on this! 🤘