Support publishing to the GitHub Package Registry

[shawnbot]

There are several ways to do this, but if we're going to support publishing to both npm and GPM (sorry) automatically we'll need to do one of the following:

1. Add instructions for publishing to GPM. Luckily, our entrypoint started as a copy of the one from actions/npm, so we already support custom values for NPM_REGISTRY_URL. In other words, all you should have to do is add this to your workflow:

   action "gpm publish" {
     uses = "primer/publish"
     env = {
       NPM_REGISTRY_URL = "<gpm registry url here>",
     }
     secrets = [
       "NPM_AUTH_TOKEN" # your GitHub access token w/necessary permissions
     ]
   }

2. Update this action to publish to GPM. We would need to add a level of abstraction around publishing to multiple registries, each with their own auth token. For instance: we might only publish to npm if NPM_AUTH_TOKEN is set, and/or to GPM if GPM_AUTH_TOKEN is set.

3. Write a new CLI that runs both of them, then call that from the action. In other words, every instance of npm publish becomes gpm publish here. I haven't looked yet, but this may already exist.

[shawnbot]

My vote would be to try option number 1 in one of our repos. Assuming GPM respects dist-tags (😬), it should just work.

[erictuvesson]

Hey, have you looked more on this @shawnbot ?

[shawnbot]

@erictuvesson Yes! @emplums started working on this over in primer/react#450, but with Actions v2 (and GPR going into public beta) we'll need to revisit that change and test it again.