Enhancement: Optional GitHub token for `self-update` API requests

The `self-update` command used only unauthenticated GitHub API requests when
checking for releases. Shared IP addresses could hit the GitHub rate limit,
resulting in a 403 Forbidden error and preventing updates.

Unauthenticated requests remain the default, but authenticated requests are now
possible. Set the environment variable `GITHUB_ACCESS_TOKEN` to a GitHub
[personal access token](https://github.com/settings/tokens) to avoid rate-limit
failures.

https://github.com/restic/restic/issues/3738
https://github.com/restic/restic/pull/5568
