Tools: use /usr/bin/env to find python3

going directly to /usr/bin/python3 means we don't use the venv python

Tools/autotest/run_mission.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 '''
 Run a mission in SITL

Tools/autotest/test_param_upgrade.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 '''
 Test parameter upgrade, master vs branch

Tools/autotest/unittest/annotate_params_unittest.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 '''
 These are the unit tests for the python script that fetches online ArduPilot

Tools/autotest/unittest/extract_param_defaults_unittest.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 '''
 Extracts parameter default values from an ArduPilot .bin log file. Unittests.

Tools/scripts/annotate_params.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 """
 This script fetches online ArduPilot parameter documentation (if not cached) and adds it to the specified file

Tools/scripts/extract_param_defaults.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 '''
 Extracts parameter default values from an ArduPilot .bin log file.

Tools/scripts/generate_pdef.xml_metadata.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 '''
 Rsync apm.pdef.xml files for different versions of the ArduPilot firmware