It's documented in the NixOS wiki that untrusted users can't set build
machines using --builders or --option builders[1]. This should cause
a warning at least to avoid confusion why the options won't take any
effect.

This applies to all options except connect-timeout, timeout and
builders if the value is empty.

[1] https://nixos.wiki/wiki/Distributed_build

I tested the behavior with the following VM declaration built from the nixpkgs master:

{
  nixpatch = { pkgs, ... }: {
    nix.package = pkgs.nix.overrideAttrs (old: {
      patches = [ ../nix/0001-nix-daemon-emit-a-warning-if-untrusted-user-tries-to.patch ];
    });

    users.extraUsers.vm = {
      password = "vm";
      isNormalUser = true;
    };
  };
}