Remove the need to explicitly pass GITHUB_TOKEN

[ezzatron]

This PR removes the need to explicitly pass a GitHub token as an action input. This helps a little to reduce the amount of configuration needed. You can still pass a different token explicitly if you want to.

GitHub's documentation on Actions doesn't really explain how to do this very clearly, but basically you have to set a default value for the token in action.yml that uses ${{ github.token }}.

For example here are some examples of GitHub's own actions that use this approach:

• https://github.com/actions/checkout/blob/2541b1294d2704b0964813337f33b291d3f8596b/action.yml#L24
• https://github.com/actions/deploy-pages/blob/791c72a9c00a798aeb2a9e602f7ff515b5624bd5/action.yml#L14
• https://github.com/actions/stale/blob/33e37032bbf25592761040e9c94772c16e52e9a4/action.yml#L8

[jdkato]

Thanks!