feat: report token permissions via env var

[jenseng]

What

Set a GITHUB_TOKEN_PERMISSIONS environment variable that reports the default github token's permissions, e.g.

{"contents": "write", "pull_requests": "read", ...}

Why

Background: https://github.com/orgs/community/discussions/73397

For authors of reusable actions, there's not a good way to know which permissions are available to the github token. Although the permissions do appear in the output (under Set up job), you can't get at them programmatically unless you do silly things like parse the Worker_*.log file to extract it from the job message 🙈

By making those permissions readable from the environment, actions can do things like:

• inform the end-user that some required permissions need to be set on the job in order for the action to work
• fall back to other strategies of accessing data, depending on which permission are available (for example)
• etc