Use GitHub App token instead of $GITHUB_ACCESS_TOKEN in the server

[joao-paulo-parity]

Generate the access token with the GitHub App instead of requiring $GITHUB_ACCESS_TOKEN in the server.

GitHub App tokens have some advantages over $GITHUB_ACCESS_TOKEN

1. Since they're generated on-demand, they don't need to be rotated
2. They automatically expire after 1h, thus making them less exploitable in the case of leaks
3. They can have fine-grained permissions
4. It's one less secret to monitor and care about, thus simplifying the deployment procedure

[mordamax]

Using GHA secret instead