nixos/promtail: check-syntax in preStart

Co-authored-by: Pol Dellaiera <pol.dellaiera@protonmail.com>

nixos/modules/services/logging/promtail.nix

@@ -41,6 +41,10 @@ in {
       wantedBy = [ "multi-user.target" ];
       stopIfChanged = false;
 
+      preStart = ''
+        ${lib.getExe pkgs.promtail} -config.file=${prettyJSON cfg.configuration} -check-syntax
+      '';
+
       serviceConfig = {
         Restart = "on-failure";
         TimeoutStopSec = 10;