Forcing unstable does not work well with setting nix paths, separate that into its own nix file

nixos/modules/legacy.nix

@@ -0,0 +1,16 @@
+{
+  # This will add each flake input as a registry
+  # To make nix commands consistent with your flake
+  nix.registry = (lib.mapAttrs (_: flake: {inherit flake;})) ((lib.filterAttrs (_: lib.isType "flake")) inputs);
+
+  # This will additionally add your inputs to the system's legacy channels
+  # Making legacy nix commands consistent as well, awesome!
+  nix.nixPath = ["/etc/nix/path"];
+  environment.etc =
+    lib.mapAttrs'
+    (name: value: {
+      name = "nix/path/${name}";
+      value.source = value.flake;
+    })
+    config.nix.registry;
+}