systemd: set dontCheckForBrokenSymlinks

[ConnorBaker]

Followup for #370750.

Should fix #370750 (comment).

Thank you for all the systemd help, @jmbaur!

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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[jmbaur]

My bad, this is actually not the right fix. In this case, this broken symlink (to /etc/environment) is intentional by systemd. The symlink is created as a backward compat to the environment.d tooling (see the bottom of https://www.freedesktop.org/software/systemd/man/latest/environment.d.html#Description). So in the sandbox, the symlink would be broken, but at runtime it might not be. Stock NixOS does not populate /etc/environment, however users would be free to do this, so we should not break that and go against what is stated in the official documentation.

[ConnorBaker]

Closed in favor of #376261.