workflows/backport: Copy security label in backport PRs

[mweinelt]

Since v1.2.0 the backport action supports copying labels from the source to the backport PR. This is useful for copying the security label from the original PR to the backport PR, so relevant security updates don't get lost.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[lheckemann]

Wooo!

I'm happy to merge this as is, but maybe we should copy all severities?

[mweinelt]

I can't remember having seen the mass rebuild severity assign anywhere. And the blocker label is usually only assigned to issues, which then appear on s.n.o.

I think scratching the security itch now is paramount. Everything else can come later, when we deem it necessary.