Add default GitHub token

[jawnsy]

If you define an action input for the GitHub token, you can include it by default, so that users don't need to write boilerplate like:

env:
  GITHUB_TOKEN: ${{ github.token }}

For an example, see https://github.com/styfle/cancel-workflow-action/blob/034d0e91921de4e82c4b8c958df266ca615543e8/action.yml#L15-L18 and https://github.com/peter-evans/create-issue-from-file/blob/ba0d2ca88c3d42f4e20ad4f1018fdb100f250b42/action.yml#LL4C1-L6C33

[mre]

Interesting! I did not know that. Can you take a look at #196 to see if it looks fine?

[jawnsy]

Looks good to me! I don't know if there's a way to check that lychee is actually picking up the token (e.g. with verbose logging or similar), that would be the only thing I'd suggest just to make sure everything is working as we think it is :)