redis: prevent infinite recursion when port is set to an env var

Modules should take care not to conditionally set `env` using
user-provided input. If the conditional is another `env` value,
evaluation will fail with an infinite recursion error.

While the user can often easily break the loop on their end, knowing
what to do requires considerable experience with the module system.

Fixes #1440.

src/modules/services/redis.nix

@@ -76,7 +76,8 @@ in
 
     env = {
       REDISDATA = config.env.DEVENV_STATE + "/redis";
-    } // optionalAttrs (cfg.port == 0) { inherit REDIS_UNIX_SOCKET; };
+      REDIS_UNIX_SOCKET = if cfg.port == 0 then REDIS_UNIX_SOCKET else null;
+    };
 
     processes.redis = {
       exec = "${startScript}/bin/start-redis";