Fix test-matrix shebang for ansible 6

Ansible 6 interprets the shebang for module files to determine which
version of python to use. Because we use env instead of python as the
executable we break all that. We run this script in testing directly as
well as via ansible as a module. That means we need a working shebang
and can't just drop it as zuul-jobs did.

Change-Id: I9a331d508276d9abe72c789fd91fc77a4136c5ca

roles/test-matrix/library/test_matrix.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/python
 
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.