[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>