openssh: set enableParallelChecking = false

this should hopefully make the tests more reliable

pkgs/tools/networking/openssh/common.nix

···
   hardeningEnable = [ "pie" ];
 
   doCheck = true;
+  enableParallelChecking = false;
   checkInputs = optional (!stdenv.isDarwin) hostname;
   preCheck = ''
     # construct a dummy HOME