main: Allow setting development mode from environment

- Similar to freedombox-privileged daemon. It makes it easy to turn on
development mode from a systemd service override (without having to override the
entire command line).

Tests:

- With the changes to systemd service file in this patch series, changing source
code file leads to plinth getting restarted.

Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/__main__.py

@@ -3,6 +3,7 @@
 
 import argparse
 import logging
+import os
 import sys
 import threading
 
@@ -121,7 +122,8 @@ def main():
     arguments = parse_arguments()
 
     cfg.read()
-    if arguments.develop:
+    if arguments.develop or os.getenv('FREEDOMBOX_DEVELOP', '') == '1':
+        cfg.develop = True
         # Use the config in the current working directory
         cfg.read_file(cfg.get_develop_config_path())