nixos/syncthing: fix declarative init crash on HTTPS

A previous config may be present that has HTTPS for the REST API enabled.

nixos/modules/services/networking/syncthing.nix

···
     do sleep 1; done
 
     curl() {
-        ${pkgs.curl}/bin/curl -sS -H "X-API-Key: $api_key" \
+        ${pkgs.curl}/bin/curl -sSLk -H "X-API-Key: $api_key" \
             --retry 1000 --retry-delay 1 --retry-all-errors \
             "$@"
     }