This required changing an excessive number of places since flakes code is a delicious bowl of copy pasta. I didn't change all of the usage sites since some of them run things on the local machine and you actually want it to be the real system there. Nevertheless, we probably should have the daemon do something much different when it receives a `system` setting: https://git.lix.systems/lix-project/lix/issues/694 Fixes: https://git.lix.systems/lix-project/lix/issues/692 Fixes: https://git.lix.systems/lix-project/lix/issues/673 Fixes: https://github.com/NixOS/nix/issues/11359 Change-Id: I55e696c09794d2520b60238a84829c98fcad7ccc