gnomeExtensions.system-monitor: fix order for collisions

[nekowinston]

#461080 changed my pkgs.gnomeExtensions.system-monitor from https://extensions.gnome.org/extension/6807/system-monitor/ -> https://extensions.gnome.org/extension/8272/system-monitor/, which doesn't indicate support for GNOME 49 yet.
This PR fixes the order and adds a new name for the other collision.

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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[nekowinston]

Looks like GIRepository.Repository is used in a bunch of places, and AFAICT Repository.dup_default() is the new way of accessing it: https://gjs-docs.gnome.org/girepository30~3.0/girepository.repository#function-dup_default