sentinel-dynfw-client: fix location for server key

By default dynfw-client is looking for /tmp/sentinel_server.key, but init script creates it in /tmp/run/sentinel_server.key which was introduced by https://gitlab.labs.nic.cz/turris/turris-os-packages/merge_requests/162.

root@turris:~# sentinel-dynfw-client 
Traceback (most recent call last):
  File "/usr/bin/sentinel-dynfw-client", line 240, in <module>
    main()
  File "/usr/bin/sentinel-dynfw-client", line 221, in main
    socket = create_zmq_socket(context, args.cert)
  File "/usr/bin/sentinel-dynfw-client", line 108, in create_zmq_socket
    server_public, _ = zmq.auth.load_certificate(server_public_file)
  File "/certs.py", line 91, in load_certificate
OSError: Invalid certificate file: /tmp/sentinel_server.key