Change shebangs of makecatalogs and repoclean to reference munki-python.

This will reduce the support burden when Apple's Python is removed in macOS 12.3.
People running these tools on non-macOS platforms will need to edit the shebang lines or call via their preferred Python.

code/client/makecatalogs

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/local/munki/munki-python
 # encoding: utf-8
 #
 # Copyright 2009-2021 Greg Neagle.

code/client/repoclean

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/local/munki/munki-python
 # encoding: utf-8
 #
 # Copyright 2016-2021 Greg Neagle.