gap: ship Makefile.gappkg for future gap package use

[collares]

We copy the testsuite to the outdir (#192548 (comment)) for installCheck purposes, but one test symlinks to etc/Makefile.gappkg, which we don't currently ship. I could delete the symlink, but the file is pretty small and I think upstream wants distros to ship it.

@fingolfin is it intentional that GAP's make install does not install etc/Makefile.gappkg? I know many GAP packages ship with a copy of this file, but I thought the idea was to use an installed version if available.

Fixes a build failure caused by #370750.

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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.

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