scripts/minikconf: Explicit usage of Python 3

Use the program search path to find the Python 3 interpreter.

Patch created manually after running:

  $ git grep -l 'if __name__.*__main__' \
      | xargs grep -LF '#!/usr/bin/env python3'

Reported-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200130163232.10446-7-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>