use github.token internally to avoid the need to pass in github_token input

[activescott]

Although it says you /can/ do this in the docs and in the github.context docs and I noticed in examples for actions created by github they don't do it such as this: https://github.com/actions/labeler/blob/main/src/labeler.ts#L16

github.context.token

[activescott]

I tried to do this in #21 by accessing the process.env["GITHUB_TOKEN"] within the action and it wasn't available in the environment like this.

I noticed some actions created by Github did this but those were old versions so maybe it doesn't work anymore?

It's also not in the type definitions of the @actions/github toolkit package for actions so looks like it is entirely unavailable or they're at least discouraging its use this way.