systemd: disable pacret hardening flag if withLibBPF

[risicle]

..for the same reason as other flags mentioned here.

Really we should not be using a wrapped clang for bpf compilation. See the #323869 saga for more information.

Without this change, systemd fails to build on aarch64-linux in the pkgsExtraHardening packageset.

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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[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/5388

[arianvp]

Thanks! Is there a tracking issue for the wrapped clang stuff?

[risicle]

I don't think so, a lot of discussion spun out @ #318674 & #323869, but most people appear to have put more effort into suppressing the warnings I added than addressing the actual problem.