Could not use plugin if GITHUB_TOKEN defined in global env

[Demonian]

I believe it is pretty standard case when GITHUB_TOKEN will be defined globally for all steps, but currently this plugin will fail in such cases with this error:

Error: Error: [@octokit/auth-action] `GITHUB_TOKEN` variable is set on both `env:` and `with:`. Use either the one or the other. See https://github.com/octokit/auth-action.js#createactionauth

It seems that making `GITHUB_TOKEN optional input parameter to fix the issue

Regards

[thisismydesign]

@Demonian Thanks for the issue. If you can open a PR I'm happy to accept it