common: move to modules, move some files out, modules: adjust dir structure

modules/common/nix.nix

@@ -0,0 +1,18 @@
+{ config, ... }:
+
+{
+  nix = {
+    channel.enable = false;
+    gc.automatic = true;
+    optimise = {
+      automatic = true;
+      dates = [ "weekly" ];
+    };
+    settings = {
+      experimental-features = [ "nix-command" "flakes" ];
+      download-buffer-size = 524288000;
+    };
+  };
+
+  nixpkgs.config.allowUnfree = true;
+}