Extract shared NixOS/Darwin base config into roles/common.nix

Create roles/common.nix containing shared configuration between NixOS and
Darwin: timezone, base packages (git, glances, pciutils, tree, usbutils, vim),
nix settings (experimental-features, max-jobs, trusted-users), gc config,
and allowUnfree setting.

Both roles/default.nix and roles/darwin.nix now import common.nix and only
contain platform-specific configuration.

roles/common.nix

@@ -0,0 +1,35 @@
+# Common configuration shared between NixOS and Darwin
+{ lib, pkgs, ... }:
+
+{
+  config = {
+    time.timeZone = "America/Los_Angeles";
+
+    environment.systemPackages = with pkgs; [
+      git
+      glances
+      pciutils
+      tree
+      usbutils
+      vim
+    ];
+
+    nix = {
+      package = pkgs.nix;
+      settings = {
+        experimental-features = [ "nix-command" "flakes" ];
+        max-jobs = "auto";
+        trusted-users = [ "johno" ];
+        substituters = [
+        ];
+      };
+
+      gc = {
+        automatic = true;
+        options = "--delete-older-than 10d";
+      };
+    };
+
+    nixpkgs.config.allowUnfree = true;
+  };
+}