clan-config: use pinned nixpkgs + fix test flake

pkgs/clan-cli/tests/machine_flake/nixosModules/machine1.nix

@@ -0,0 +1,7 @@
+{ lib, ... }: {
+  options.clan.jitsi.enable = lib.mkOption {
+    type = lib.types.bool;
+    default = false;
+    description = "Enable jitsi on this machine";
+  };
+}