Expand the scope of the `allowUnfree` flag to all the inputs

[sestrella]

Right now, it appears that setting allowUnfree = true only affects the nixpkgs input; a closer look at the generated .devenv.flake.nix file reveals the following code:

pkgs = import nixpkgs {
  inherit system;
  config = {
    allowUnfree = devenv.allowUnfree or false;
    allowBroken = devenv.allowBroken or false;
    permittedInsecurePackages = devenv.permittedInsecurePackages or [ ];
  };
  inherit overlays;
};

So far, it appears that the only way to install "unfree" packages from external flakes is to pass them as overlays to the main nixpkgs input, which may be affected by allowUnfree = true. A workaround I discovered is to prefix all devenv commands with NIXPKGS_ALLOW_UNFREE=1 or to add it directly to the direnv configuration file .envrc, as shown below:

source_url "https://raw.githubusercontent.com/cachix/devenv/95f329d49a8a5289d31e0982652f7058a189bfca/direnvrc" "sha256-d+8cBpDfDBj41inrADaJt+bDWhOktwslgoP5YiGJ1v0="

NIXPKGS_ALLOW_UNFREE=1 use devenv

To avoid this workaround, I am wondering if it makes sense to pass the NIXPKGS_ALLOW_UNFREE=1 environment variable to all nix commands invoked by devenv when allowUnfree is set to true. I'd be willing to submit a PR if the suggested change sounds like a viable solution for the mentioned scenario.