refactor: Move nix outside of .config dir

Technically not a config but a compose file, so I think it's better to
have outside of `.config` dir

nix/lib/mkSystem.nix

@@ -0,0 +1,29 @@
+{ arch, stable, unstable, nur, extraOverlays ? [] }:
+
+let
+  overlays = [
+      (import ../overlays/python.nix)
+  ] ++ extraOverlays;
+
+  # Placed here so that we don't need to specify hash for fetchTarball
+  packageOverrides = pkgs: {
+    nur = import nur {
+      nurpkgs = pkgs;
+      inherit pkgs;
+    };
+  };
+in {
+  system = arch;
+  pkgs = import stable {
+    system = arch;
+    inherit overlays;
+    config.allowUnfree = true;
+    config.packageOverrides = packageOverrides;
+  };
+  pkgs-unstable = import unstable {
+    system = arch;
+    inherit overlays;
+    config.allowUnfree = true;
+    # config.packageOverrides = packageOverrides;
+  };
+}