forward: allow reading secret from environment variable to prevent argv exposure

When using `gh webhook forward --secret `, the secret is passed via the command line arguments (argv). This makes it visible to process listing tools (such as Task Manager on Windows, or `ps` on Linux) and exposes it to other processes running under the same user.

It would be safer to allow passing the webhook secret via an environment variable (e.g. `GH_WEBHOOK_SECRET`) so that it doesn't leak into the process list.