software/fluentd/test: use sys.executable instead of #!/usr/bin/python2.7

On modern systems it is not installed and anyway slapos should not
depend on host system programs.
Basically same fix as b964b95b (software/fluentd: Fix SR test on Debian
11, 2022-01-04)

software/fluentd/test/test.py

@@ -190,11 +190,12 @@ class SensorConfTestCase(WendelinTutorialTestCase):
 
   @classmethod
   def sensor_script(cls, measurementList):
-    return '''\
-#!/usr/bin/python
+    measurement_text = "\t".join(measurementList)
+    return f'''\
+#!{sys.executable}
 # -*- coding: utf-8 -*-
 
-print("%s")''' % "\t".join(measurementList)
+print("{measurement_text}")'''
 
   def test_configuration(self):
     self._test_configuration(