Use the right python when starting workers

We should use the same python to start the workers as we do for the main
synapse (ie, the same one used to run synctl.)

changelog.d/4057.bugfix

@@ -0,0 +1 @@
+synctl will use the right python executable to run worker processes

synctl

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 # Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -86,7 +87,7 @@ def start(configfile):
 
 def start_worker(app, configfile, worker_configfile):
     args = [
-        "python", "-B",
+        sys.executable, "-B",
         "-m", app,
         "-c", configfile,
         "-c", worker_configfile