programs.singularity.packageOverriden

This option provides access to the overridden result of programs.singularity.package.

For example, the following configuration makes all the Nixpkgs packages use the overridden singularity:

{ config, lib, pkgs, ... }:
{
  nixpkgs.overlays = [
    (final: prev: {
      _singularity-orig = prev.singularity;
      singularity = config.programs.singularity.packageOverriden;
    })
  ];
  programs.singularity.enable = true;
  programs.singularity.package = pkgs._singularity-orig;
}

Use lib.mkForce to forcefully specify the overridden package.