Add shared modules for nix-gc and nix-settings

modules/nix-gc.nix

@@ -0,0 +1,7 @@
+_: {
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 60d";
+  };
+}

modules/nix-settings.nix

@@ -0,0 +1,15 @@
+_: {
+  nix.settings = {
+    extra-experimental-features = [
+      "nix-command"
+      "flakes"
+    ];
+
+    auto-optimise-store = true;
+
+    trusted-users = [ "@wheel" ];
+
+    # RPi builds can be slow due to compiling via binfmt.
+    download-buffer-size = 1 * 1024 * 1024 * 1024;
+  };
+}