blinkd: Use system python

This is the second part of the blinkd fix (issue #54).

PR #64 prevented the deadlock during service start.

However, currently blinkd is not starting at system boot, because
it uses `env python3` in the shebang. This will activate our default
(big, scientific) python3 which is located in /pkg and depends
on the automounter.

One solution to this problem would be to add After=automount.service
to blinkd.service. This would delay the startup of blinkd to the
point, when automounter has finished startup. However, experiments
showed, that this doesn't work. It looks like the startup
command of automount.service exists after daemon process has been
forked but before the automount paths have been mounted. So
systemd regards automount.service has started before the automount
paths are really available and services with After=automount.service
are still started to early to use them.

That aside, although /usr/bin/python3 depends on /pkg, it is not
supposed to go via NFS under normal circumstances, because of the
local mechanism on mxpkg. However, under exceptional circumstances
(e.g. transition to a new python3 version) it might go over
NFS and we don't want long-running deamons to lock nfs mounts.

A python3 with the blink1 package from PyPi has been added
to the system [1] and is available as
/usr/local/system/python3/bin/python3.

This python installation is guaranteed to be on a local
disk, so use this path in the shebang.

[1] mariux64/pkg-scripts@8e149c1

blink/blinkd.py

@@ -1,5 +1,4 @@
-#!/usr/bin/env python3
-
+#! /usr/local/system/python3/bin/python3
 
 from blink1.blink1 import Blink1, Blink1ConnectionFailed
 from multiprocessing import cpu_count