modified user to be modular, separated machine specific configs

modules/common/core.nix

@@ -0,0 +1,26 @@
+{
+  pkgs,
+  lib,
+  ...
+}: {
+  # enable flakes globally
+  nix.settings.experimental-features = ["nix-command" "flakes"];
+
+  # Allow unfree packages
+  nixpkgs.config.allowUnfree = true;
+
+  nix.package = pkgs.nix;
+
+  # do garbage collection weekly to keep disk usage low
+  nix.gc = {
+    automatic = lib.mkDefault true;
+    options = lib.mkDefault "--delete-older-than 7d";
+  };
+
+  # Disable auto-optimise-store because of this issue:
+  #   https://github.com/NixOS/nix/issues/7273
+  # "error: cannot link '/nix/store/.tmp-link-xxxxx-xxxxx' to '/nix/store/.links/xxxx': File exists"
+  nix.settings = {
+    auto-optimise-store = false;
+  };
+}