Adds a new --use-prebuilt-closure option to nixos-rebuild-ng, which will avoid building the new system closure and instead use the prebuilt one specified.

I wanted this feature since I often nix copy --from closures from my build machine and want to use the proper nixos-rebuild script to switch into that closure. I could also see this being useful to activate old generations of closures.

As a quick sanity check, you can run something like the following to test this new feature:

sudo ./result/bin/nixos-rebuild-ng --use-prebuilt-closure /run/current-system/ switch

which of course, will just reactivate your currently running closure.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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