stdenv/check-meta: add maxSilent

Hydra supports it
https://github.com/NixOS/hydra/blob/master/src/hydra-eval-jobs/hydra-eval-jobs.cc#L172

pkgs/stdenv/generic/check-meta.nix

@@ -231,6 +231,7 @@ let
     timeout = int;
 
     # Weirder stuff that doesn't appear in the documentation?
+    maxSilent = int;
     knownVulnerabilities = listOf str;
     name = str;
     version = str;