Stop assuming that /usr/bin/python exists

Fixes 33192

scripts/codegen/fuzzing_include_am.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 # Future imports for Python 2.7, mandatory in 3.0
 from __future__ import division

scripts/codegen/gen_server_ciphers.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2014-2019, The Tor Project, Inc
 # See LICENSE for licensing information
 

scripts/codegen/get_mozilla_ciphers.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # coding=utf-8
 # Copyright 2011-2019, The Tor Project, Inc
 # original version by Arturo Filastò

scripts/codegen/makedesc.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2014-2019, The Tor Project, Inc.
 # See LICENSE for license information