maintainers: add targeted fix for missing pkgs/by-name/.../package.nix files

[philiptaron]

Motivation for changes

• maintainers/scripts/build.nix isn't ignoring empty dirs in pkg/by-name #338227

Fixes #338227.

Description of changes

This is my attempt at a more targeted fix for the issue described in #338227. @cafkafk proposed a fix in #340778 which has performance implications.

• Added an overlay to the pkgs instantiation in maintainers/scripts/build.nix
• Might need to add it to others and make common if this problem shows up in other places.

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.

[cafkafk]

LGTM, thanks!

[infinisil]

Nice, thanks!