Light refactor, enable parallelism for nix building, builders use substitute and make sure darwin has the same benefits as linux

nix/system/common/nix.nix

@@ -0,0 +1,27 @@
+{ pkgs, ... }:
+{
+  environment.systemPackages = with pkgs; [
+    vim
+  ];
+
+  nix = {
+    settings = {
+      experimental-features = "nix-command flakes";
+      auto-optimise-store = true;
+      max-jobs = "auto";
+      cores = 0;
+
+      builders-use-substitutes = true;
+    };
+
+    extraOptions = ''
+      keep-going = true
+    '';
+
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
+  };
+}

nix/system/linux.nix

@@ -37,17 +37,6 @@
     };
   };
 
-  nix = {
-    settings = {
-      auto-optimise-store = true;
-    };
-    gc = {
-      automatic = true;
-      dates = "weekly";
-      options = "--delete-older-than 7d";
-    };
-  };
-
   services = {
     pipewire = {
       enable = true;