userns-block-fd: Search $PATH for python

On systems without the /usr merge, it's almost certainly in /usr,
so this script would have failed.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #246
Approved by: giuseppe

demos/userns-block-fd.py

@@ -1,4 +1,4 @@
-#!/bin/python
+#!/usr/bin/env python
 
 import os, select, subprocess, json