Default to GitHub token

[henryiii]

The GitHub token can be set as default (wasn't possible in the distant past, I think, but now is and several of the core actions/ now do this, along with a growing number of 3rd party actions). This would make the action a bit less verbose for normal use.

See https://github.com/actions/checkout/blob/5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f/action.yml#L24 for example.

[styfle]

Good idea! Do you want to create a PR?