github_access_token not accepted on v18

The README suggests the use of a `github_access_token` input with v18 of the action but the following error occurs:

```
Warning: Unexpected input(s) 'github_access_token', valid inputs are ['install_url', 'install_options', 'nix_path', 'extra_nix_config']
```

I think the action just needs another release, perhaps v19.