nixos/kresd: set .stopIfChanged = false

Since version 5.2.0 there's non-empty stop phase:
    ExecStopPost=/usr/bin/env rm -f "/run/knot-resolver/control/%i"
but it's perfectly OK to run that from a different version
(and typically it's no-op anyway).  Real-life example where this helps:
https://github.com/NixOS/nixpkgs/issues/49528#issuecomment-747723198

nixos/modules/services/networking/kresd.nix

@@ -135,6 +135,8 @@ in {
       CacheDirectory = "knot-resolver";
       CacheDirectoryMode = "0770";
     };
+    # We don't mind running stop phase from wrong version.  It seems less racy.
+    systemd.services."kresd@".stopIfChanged = false;
 
     # Try cleaning up the previously default location of cache file.
     # Note that /var/cache/* should always be safe to remove.