nixpkgs.config: Add allowUnfreePackages option

Also make sure it merges correctly and verify with a test.

This was added in nixpkgs some time ago and is quite a nice addition as
it merges additively (in contrast to allowUnfreePredicate) thus allowing
to localize allowUnfreePackage specifications to all the locations in
your local config where unfree packages are added.

tests/nixpkgs-config-allow-unfree.nix

@@ -0,0 +1,45 @@
+# Check that nixpkgs.config.allowUnfreePackages is merged correctly
+# run with: nix-build release.nix -A tests.nixpkgs-config-allow-unfree
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  # Module 1: Define some unfree packages
+  nixpkgs.config.allowUnfreePackages = [
+    "vscode"
+    "slack"
+  ];
+
+  # Module 2: Define more unfree packages (simulating multiple modules)
+  # In a real scenario, this would be in a separate file
+  imports = [
+    (
+      { config, ... }:
+      {
+        nixpkgs.config.allowUnfreePackages = [
+          "zoom"
+          "discord"
+        ];
+      }
+    )
+  ];
+
+  test = ''
+    echo checking allowUnfreePackages merging >&2
+
+    # Verify that all packages from both modules are present
+    expected_packages=("discord" "slack" "vscode" "zoom")
+    actual_packages=(${builtins.toString (builtins.sort builtins.lessThan config.nixpkgs.config.allowUnfreePackages)})
+
+    for pkg in "''${expected_packages[@]}"; do
+      if [[ ! " ''${actual_packages[@]} " =~ " $pkg " ]]; then
+        echo "ERROR: Expected package '$pkg' not found in allowUnfreePackages" >&2
+        exit 1
+      fi
+    done
+  '';
+}