'require' does not work on flakes deployments

[cab404]

To repro:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    nixops.url = "github:nixos/nixops";
  };

  outputs = { self, nixpkgs, nixops, ... }: {
    nixopsConfigurations = {
      default = {
        inherit nixpkgs;
        require = [


          # Won't trigger any errors, or affect config if exists.
          ./this_file_does_not_exist.nix 


        ];
        host = { };
      };
    };

    # for ease of testing
    devShell.x86_64-linux = with nixpkgs.legacyPackages.x86_64-linux; mkShell {
      buildInputs = [ nixops.defaultPackage.x86_64-linux ];
    };

  };

}

[roberth]

We should consider replacing require first: #1486