GRD 50.x invokes pkexec internally for every grdctl --system call, even
when the caller is root. An isolated systemd script PATH does not include
/run/wrappers/bin automatically, causing exit 70 at boot.
Changes:
- Prepend /run/wrappers/bin to PATH at script start so every subsequent
grdctl --system resolves the NixOS setuid pkexec wrapper.
- Add an explicit preflight check (test -x /run/wrappers/bin/pkexec) with
a clear error message before the first grdctl_system call.
- pkgs.polkit remains absent from the service path.
- Update test_setup_runs_grdctl_directly_as_root to reflect that pkexec
now appears in the preflight check (not as a direct invocation).
- Add test_run_wrappers_bin_prepended_to_path and test_pkexec_preflight_check.