Add nix configuration

Make wheel group trusted users in preparation for colmena.

config/common/default.nix

@@ -5,6 +5,7 @@
     ./default-host-platform.nix
     ./default-state-version.nix
     ./localization.nix
+    ./nix.nix
     ./ssh.nix
     ./users.nix
   ];

config/common/nix.nix

@@ -0,0 +1,16 @@
+{ ... }: 
+
+{
+  nix = {
+    settings = {
+      trusted-users = [ "@wheel" ];
+      auto-optimise-store = true;
+      experimental-features = [ "nix-command" "flakes" ];
+    };
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 30d";
+    };
+  };
+}