nixos/lemmy: only use env var when instructed

Lemmy checks the environment variable before the configuration file;
i.e. if the file is used to configure the database but the environment
variable is set to anything, the connection will fail because it'll
ignore the file. This was the previous behavior.

Now, the environment variable will be unset unless the user explicitly
chooses to set it, which makes the file-based configuration function
correctly. It's also possible to manually set the environment variable,
which has the major advantage of working around [this issue][0], which
prevents certain setups from working.

[0]: LemmyNet/lemmy#2945

nixos/modules/services/web-apps/lemmy.nix

@@ -27,7 +27,15 @@ in
     caddy.enable = mkEnableOption (lib.mdDoc "exposing lemmy with the caddy reverse proxy");
     nginx.enable = mkEnableOption (lib.mdDoc "exposing lemmy with the nginx reverse proxy");
 
-    database.createLocally = mkEnableOption (lib.mdDoc "creation of database on the instance");
+    database = {
+      createLocally = mkEnableOption (lib.mdDoc "creation of database on the instance");
+
+      uri = mkOption {
+        type = with types; nullOr str;
+        default = null;
+        description = lib.mdDoc "The connection URI to use. Takes priority over the configuration file if set.";
+      };
+    };
 
     settings = mkOption {
       default = { };
@@ -190,9 +198,7 @@ in
 
         environment = {
           LEMMY_CONFIG_LOCATION = "/run/lemmy/config.hjson";
-
-          # Verify how this is used, and don't put the password in the nix store
-          LEMMY_DATABASE_URL = with cfg.settings.database;"postgres:///${database}?host=${host}";
+          LEMMY_DATABASE_URL = mkIf (cfg.database.uri != null) cfg.database.uri;
         };
 
         documentation = [