check-meta.nix: fix checkMetaRecursively option

In specific cases, combining the `checkMeta` and `checkMetaRecursively`
config options would result in `error: infinite recursion encountered`

fixes #193296

pkgs/stdenv/generic/check-meta.nix

@@ -298,7 +298,7 @@ let
     executables = listOf str;
     outputsToInstall = listOf str;
     position = str;
-    available = bool;
+    available = unspecified;
     isBuildPythonPackage = platforms;
     schedulingPriority = int;
     isFcitxEngine = bool;