[fix] follow up of PR-1856

- the environment variable SEARXNG_REDIS_URL overrides the setting value redis.url
- ./manage sets SEARXNG_REDIS_URL to unix:///usr/local/searxng-redis/run/redis.sock if:
   - the socket exists
   - SEARXNG_REDIS_URL is not already defined

Update of PR #1856

Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings_defaults.py

@@ -174,7 +174,7 @@ SCHEMA = {
         'default_http_headers': SettingsValue(dict, {}),
     },
     'redis': {
-        'url': SettingsValue((None, False, str), False),
+        'url': SettingsValue((None, False, str), False, 'SEARXNG_REDIS_URL'),
     },
     'ui': {
         'static_path': SettingsDirectoryValue(str, os.path.join(searx_dir, 'static')),