nix flake check: log success in verbose mode

The rule of silence can be a little surprising. As a compromise to
changing the default behavior, this adds printing a success message in
verbose mode, where we don't really have a reason to be silent about
our success.

Author: CyberShadow

src/nix/flake.cc

@@ -811,6 +811,8 @@ struct CmdFlakeCheck : FlakeCommand
         if (hasErrors)
             throw Error("some errors were encountered during the evaluation");
 
+        logger->log(lvlInfo, ANSI_GREEN "all checks passed!" ANSI_NORMAL);
+
         if (!omittedSystems.empty()) {
             // TODO: empty system is not visible; render all as nix strings?
             warn(