Automatically tag security-related pull requests

[pacien]

Automatically add the "security" label on pull requests that contain patterns like CVE-*-* in their title or in a commit message.

[FRidh]

This is now especially interesting with the introduction of CVE reporting by @r-ryantm https://discourse.nixos.org/t/nixpkgs-update-partners-with-serokell-and-nlnet-to-add-cve-reporting/3577/5

[wamserma]

ofborg should also add the security label when the PR references Issues that have the security label. e.g. (at the time of writing this comment) here: https://github.com/NixOS/nixpkgs/pull/100441