nix settings: unify for NixOS and nix-darwin

moved into a common module

common/nix-settings.nix

@@ -0,0 +1,34 @@
+# needs to be kept compatible to both NixOS and nix-darwin
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+
+let
+  inputs = config.inputInjection.flake-inputs;
+  unstable = inputs.nixos-unstable;
+in
+{
+  nix = {
+    # expose all flake inputs through nix Path and registry
+    registry = (lib.mapAttrs (_: value: { flake = value; }) inputs) // {
+      nixpkgs.flake = inputs.nixpkgs;
+    };
+    nixPath = lib.mapAttrsToList (key: value: "${key}=${value.to.path}") config.nix.registry;
+  };
+  nix.settings = builtins.mapAttrs (_: lib.mkDefault) {
+    # keep around all inputs necessary for offline-rebuilding the system
+    keep-outputs = true;
+    keep-derivations = true;
+    trusted-users = [ "spiollinux" ];
+    experimental-features = [
+      "nix-command"
+      "flakes"
+      "repl-flake"
+    ];
+    # use all cores for building
+    cores = 0;
+  };
+}