PUT api/machines/{name}/config: ensure only valid config is ever written

- add CLAN_MACHINE_SETTINGS_FILE variable to temporarily override the machine settings file
- do a dry-run evaluation first with the new config before persisting it.

pkgs/clan-cli/tests/test_flake/flake.nix

@@ -6,9 +6,13 @@
     nixosConfigurations.machine1 = inputs.nixpkgs.lib.nixosSystem {
       modules = [
         ./nixosModules/machine1.nix
-        (if builtins.pathExists ./machines/machine1/settings.json
-        then builtins.fromJSON (builtins.readFile ./machines/machine1/settings.json)
-        else { })
+        (
+          if builtins.getEnv "CLAN_MACHINE_SETTINGS_FILE" != ""
+          then builtins.fromJSON (builtins.readFile (builtins.getEnv "CLAN_MACHINE_SETTINGS_FILE"))
+          else if builtins.pathExists ./machines/machine1/settings.json
+          then builtins.fromJSON (builtins.readFile ./machines/machine1/settings.json)
+          else { }
+        )
         ({ lib, options, pkgs, ... }: {
           config = {
             nixpkgs.hostPlatform = "x86_64-linux";