fix: switch shebang from hardcoded /usr/bin/python to portable /usr/bin/env python

Replace the explicit /usr/bin/python interpreter path with /usr/bin/env python
in generate_corelib.py and generate_docs.py scripts, allowing the system to
use the first python found in PATH instead of a fixed installation location.

script/generate_corelib.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 import re
 
 with open("corelib.wren", "r") as f:

script/generate_docs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 import glob
 import markdown