Updated CI release workflow to use built-in token
Updated CI release workflow to use built-in token

no issue

- `RELEASE_TOKEN` is currently a GitHub personal token, but this
  has some downsides:
    - if the token ever expires and I'm unaware, it'll break the release
      process
    - GitHub Releases say the creator was `daniellockyer` even if someone
      else actually did the release
- this commit switches over to using the built-in `GITHUB_TOKEN`, which
  is owned by the `github-actions` app and should never expire
- aside from that, Ghost releases will be created by the neutral
  `github-actions` account

janvt/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost,ErisDS/Ghost,TryGhost/Ghost,TryGhost/Ghost,e10/Ghost,e10/Ghost,ErisDS/Ghost