yarn2nix: avoid appearing to include src

[philiptaron]

Description of changes

This expression didn't actually include src, but it looked like it did. Make it like others by using a fileset (for instance, #319883)

I verified that this didn't do anything by diffing the two derivation and seeing no difference.

diff -r -u4 /nix/store/sf2zmfydysqpk38nwa84yv41hcprby6m-yarn2nix /nix/store/wlvsdkmglkq6n63akvnhg929rllxwv68-source

• [Tracking issue] remove all src = ./. entries that unintentionally include their own .nix files #301014

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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.

[infinisil]

Really nice to see the kind of mess we can avoid with lib.fileset :D