Add common settings to NixOS and nix-darwin configurations

These are sensible defaults we generally want across all configurations.

nix/modules/flake-parts/lib.nix

@@ -22,6 +22,25 @@ let
         }
       ];
     };
+
+    # Common and useful setting across all platforms
+    common = { flake, lib, ... }: {
+      nix = {
+        # Enables use of `nix-shell -p ...` etc
+        nixPath = [ "nixpkgs=${flake.inputs.nixpkgs}" ];
+        # Make `nix shell` etc use pinned nixpkgs
+        registry.nixpkgs.flake = flake.inputs.nixpkgs;
+
+        settings = {
+          # Use all CPU cores
+          max-jobs = lib.mkDefault "auto";
+          # Duh
+          experimental-features = lib.mkDefault "nix-command flakes";
+          # Nullify the registry for purity.
+          flake-registry = builtins.toFile "empty-flake-registry.json" ''{"flakes":[],"version":2}'';
+        };
+      };
+    };
   };
 
   darwinModules = {
@@ -67,6 +86,7 @@ in
           specialArgs = specialArgsFor.nixos;
           modules = [
             ../configurations
+            nixosModules.common
             mod
           ] ++ lib.optional home-manager nixosModules.home-manager;
         };
@@ -75,6 +95,7 @@ in
           specialArgs = specialArgsFor.darwin;
           modules = [
             ../configurations
+            nixosModules.common
             darwinModules.nix-darwin
             mod
           ] ++ lib.optional home-manager darwinModules.home-manager;