gnomeExtensions.unite: remove manually packaging and use extension override

[rhoriguchi]

Motivation for this change

Depends on #140285. Since the current version in extensions.json is lower than the packed one.

Use the new override logic from #137131 to add buildInputs.

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 via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 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.

[piegamesde]

Looks great, thank you. Would you mind also adding yourself to the maintainers of that package using the override?

[rhoriguchi]

Looks great, thank you. Would you mind also adding yourself to the maintainers of that package using the override?

Sure, I've added myself.