Fix home-manager impurity and add brew configuration

machines/demeter/configuration.nix

@@ -1,49 +1,12 @@
 { pkgs, ... }:
 
 {
-  imports = [ <home-manager/nix-darwin> ];
-
   nixpkgs.config.allowUnfree = true;
 
   environment = { variables = { LANG = "en_US.UTF-8"; }; };
 
   networking = { hostName = "demeter"; };
 
-  system.defaults = {
-    NSGlobalDomain = {
-      AppleMeasurementUnits = "Centimeters";
-      AppleMetricUnits = 1;
-      AppleShowScrollBars = "Automatic";
-      AppleTemperatureUnit = "Celsius";
-      AppleKeyboardUIMode = 3;
-      ApplePressAndHoldEnabled = false;
-      InitialKeyRepeat = 10;
-      KeyRepeat = 3;
-      _HIHideMenuBar = true;
-    };
-
-    dock = {
-      autohide = true;
-      mru-spaces = false;
-      tilesize = 512;
-    };
-
-    finder = {
-      AppleShowAllExtensions = true;
-      QuitMenuItem = true;
-      FXEnableExtensionChangeWarning = false;
-    };
-
-    trackpad = {
-      Clicking = true;
-      TrackpadThreeFingerDrag = true;
-      TrackpadRightClick = true;
-    };
-
-    # Login and lock screen
-    loginwindow = { GuestEnabled = false; };
-  };
-
   # Auto upgrade nix package and the daemon service.
   services.nix-daemon.enable = true;
   nix.package = pkgs.nixFlakes;