CI: Make matrix.py executable and add shebang

It's a script, allow executing this as a script without needing to invoke
`python3` manually.
diff --git a/.github/matrix.py b/.github/matrix.py
old mode 100644
new mode 100755
index 4735248..cfef53c
--- a/.github/matrix.py
+++ b/.github/matrix.py
@@ -1,3 +1,5 @@
+#!/usr/bin/python3
+
 # Copyright 2019 Ilya Shipitsin <chipitsine@gmail.com>
 # Copyright 2020 Tim Duesterhus <tim@bastelstu.be>
 #