NixOSConfig/lib/config/custom-build-vm.nix

{ config, lib, options, pkgs, ... }:
  let
    vmOptions = { };
  in {
    options = {
      virtualisation = {
        vmVariant = vmOptions;
        vmVariantWithBootLoader = vmOptions;
      };
    };

    config = {
      # Replace native `qemu` with `remote-viewer`
      system.build =
        {
          vm =
            let
              packageName = "custom-nixos-vm";

              mergedSystem =
                with options.system;
                lib.mergeDefinitions
                  build.loc
                  build.type
                  (lib.lists.forEach
                    (
                      builtins.filter
                        (item:
                          !(lib.path.hasPrefix ./. (/. + item.file)))
                        build.definitionsWithLocations)
                    (item: { inherit (item) file value; }));
            in
              lib.mkForce (
                if (mergedSystem.mergedValue.vm.name == packageName)
                then
                  mergedSystem.mergedValue.vm
                else
                  let
                    wrapped = pkgs.writeShellApplication {
                      name = "run-${config.system.name}-vm";
                      text = ''
                        ${mergedSystem.mergedValue.vm}/bin/run-${config.system.name}-vm
                      '';
                    };
                  in
                    # Rename package to `nixos-vm`
                    pkgs.symlinkJoin {
                      name = packageName;
                      paths = [ wrapped ];
                    });
        };
    };
  }