TODO

- Contribute an impl of Error back to narinfo crate.
- Add a polkit rule to be used with this:
  polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.systemd1.manage-units" && subject.user == "nixless-agent") {
      if (action.lookup("unit") == "<name of transient unit>") {
        if (action.lookup("verb") == "start") {
          return polkit.Result.YES;
        }
      }
    }
  });
- Figure out why nix is rebuilding the NixOS test machines even when other somewhat unrelated stuff changed (e.g. the test script changes, then the machines are rebuilt).