run-dev: Run process_queue with DJANGO_AUTORELOAD_ENV.

In Django 2.2 the autoreload system has changed.
DJANGO_AUTORELOAD_ENV env variable should be set when calling code
that'll use the autoreloader. Otherwise there's some kind of race
condition in the autoreload code when SIGINT is sent, where
restart_with_reloader() (called only if the env variable isn't set)
has the subprocess module calling p.kill() on a process that's already
exited, raising ProcessLookupError and printing an ugly traceback. This
causes non-deterministic test-run-dev failures.

tools/run-dev.py

@@ -10,6 +10,7 @@ import traceback
 
 from urllib.parse import urlunparse
 
+from django.utils.autoreload import DJANGO_AUTORELOAD_ENV
 # check for the venv
 from lib import sanity_check
 sanity_check.check_venv(__file__)
@@ -141,7 +142,8 @@ cmds = [['./manage.py', 'runserver'] +
         manage_args + runserver_args + ['127.0.0.1:%d' % (django_port,)],
         ['env', 'PYTHONUNBUFFERED=1', './manage.py', 'runtornado'] +
         manage_args + ['127.0.0.1:%d' % (tornado_port,)],
-        ['./manage.py', 'process_queue', '--all'] + manage_args,
+        ['env', '{}=true'.format(DJANGO_AUTORELOAD_ENV),
+         './manage.py', 'process_queue', '--all'] + manage_args,
         ['env', 'PGHOST=127.0.0.1',  # Force password authentication using .pgpass
          './puppet/zulip/files/postgresql/process_fts_updates'],
         ['./manage.py', 'deliver_scheduled_messages'],