nixos/tests/gitea: fix eval warnings

trace: warning: config.services.gitea.database.password will be stored as plaintext
        in the Nix store. Use database.passwordFile instead.

(Arguably, this shouldn't be a warning at all. But making it happy is
easier than having a debate on the value of this warning.)

nixos/tests/gitea.nix

@@ -45,7 +45,7 @@ with pkgs.lib;
       {
         services.gitea.enable = true;
         services.gitea.database.type = "postgres";
-        services.gitea.database.password = "secret";
+        services.gitea.database.passwordFile = pkgs.writeText "db-password" "secret";
       };
 
     testScript = ''