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
- The default Node version on GitHub Actions is moving to Node 14 and we
want to keep building it on 12 for now, so this commit pins the
version to 12.18.0
- we were using `ubuntu-latest`, which is an alias for 18.04, but there's an
increasingly likely chance that the default becomes 20.04 soon.
- we don't officially support 20.04 yet, so we want to pin to our
supported version until we're ready
