Based on the documentation, I have my workflow set up to install from my GitHub Package Registry:

    steps:
    - uses: actions/checkout@v1
    - uses: actions/setup-node@v1
      with:
        node-version: 10.x
        registry-url: 'https://npm.pkg.github.com'
    - run: npm ci
      env:
        NODE_AUTH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

However, I get a 404 when trying to install any private packages scoped to my account with this configuration. Just to clarify these are private packages within the same account that this repo and workflow exists.

Using the exact same configuration, if I replace with a personal access token I've created, I am able to install private packages without issue.