docs: update a couple nix usage instructions

Readme.md

@@ -27,7 +27,6 @@ We compile and cache the tools for the following platforms:
 * [Verilator](https://verilator.org)
 * [Icarus Verilog](https://github.com/steveicarus/iverilog)
 * [cocotb](https://www.cocotb.org/)
-    * Linux only.
 * [Xschem](https://xschem.sourceforge.io/stefan/index.html)
 * [Xyce](https://github.com/xyce/xyce)
     * Linux only.

docs/flake-template.nix

@@ -41,7 +41,7 @@
         })
         (
           nix-eda.composePythonOverlay (pkgs': pkgs: pypkgs': pypkgs: let
-            callPythonPackage = lib.callPackageWith (pkgs' // pkgs'.python3.pkgs);
+            callPythonPackage = lib.callPackageWith (pkgs' // pypkgs');
           in {
             # Add python package derivations here
           })

docs/flake_api.md

@@ -48,7 +48,7 @@ creation of dependent Flakes, ranked by importance:
 
   ```nix
   self.composePythonOverlay (pkgs': pkgs: pypkgs': pypkgs: let
-    callPythonPackage = lib.callPackageWith (pkgs' // pkgs'.python3.pkgs);
+    callPythonPackage = lib.callPackageWith (pkgs' // pypkgs');
   in {
     gdsfactory = callPythonPackage ./nix/gdsfactory.nix {};
   })

docs/installation.md

@@ -82,10 +82,11 @@ You can do that by adding these statements to your OS's derivation:
 ```nix
 {
   nix = {
-    extraOptions = ''
-      extra-experimental-features = nix-command flakes
-    '';
     settings = {
+      experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
       substituters = [
         "https://nix-cache.fossi-foundation.org"
       ];

nix/cocotb.nix

@@ -1,19 +1,4 @@
-# Copyright 2025 nix-eda Contributors
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-# Code adapated from nixpkgs, original license follows
-# ---
-# Copyright (c) 2003-2025 Eelco Dolstra and the Nixpkgs/NixOS contributors
+# Copyright (c) 2025 nix-eda Contributors
 #
 # Permission is hereby granted, free of charge, to any person obtaining
 # a copy of this software and associated documentation files (the