.github/workflows/labels.yml: set event types
.github/workflows/labels.yml: set event types

opened, synchronize, reopened are the defaults for `pull_request_target`,
`edited` will trigger the label action if the PRs base branch is changed.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs