systemd-cryptsetup-generator: Add note to revert 3efadce.

pkgs/os-specific/linux/systemd/cryptsetup-generator.nix

···
 
   # For some reason systemd-cryptsetup-generator is a wrapper-script
   # with the current release of systemd. We want the real one.
+
+  # TODO: Revert 3efadce when the wrapper-script is gone
   installPhase = ''
     mkdir -p $out/lib/systemd/
     cp systemd-cryptsetup $out/lib/systemd/systemd-cryptsetup