[FR] Use $GITHUB_TOKEN for API requests if available

[webknjaz]

The current implementation makes queries within a public IP-based rate limit which might be problematic on shared networks. It would be useful to use a token available as an environment variable. It's usually present as a secret within GHA so it would make it possible to run this tool from within a GHA workflow.

Point of integration: https://github.com/davidism/gha-update/blob/b752b98/src/gha_update/_core.py#L91C16-L91C62