Followup for #370750.

Given some packages purposefully create symlinks which point out of the store, it makes sense to skip them in the check rather than error or warn because the hooks is limited to sandbox.

The two additional tests I've added succeed, and systemdMinimal succeeds without disabling the hook (like in #376218, which I have closed).

This PR additionally fixes the flags passed to ln when the absolute option is provided in the tests -- previously, passing a value of true for absolute would create a relative symlink! Thank you @emilazy for spotting that!

This issue wasn't caught by the tests because they would need to build with NIX_DEBUG set to a high value and grep for a logging statement, since the hook canonicalizes relative symlinks to absolute paths when evaluating them, and the only difference in code-path is a single logging statement.

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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