Use env python interpreter in insert-includes.py

This fixes failing CI for CVA6/nvdla on a system that does not have a
/usr/bin/python interpreter by making the script use one from conda env.

scripts/insert-includes.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 # replaces a `include with the full include file
 #