Skip parallel/taskPar/private on cygwin due to hangs.

`test/parallel/taskPar/sungeun/private.chpl` intermittently hangs in nightly cygwin testing. Normally, this would just result in a timeout, but the timedexec script is (for some reason unknown to me) unable to kill the test. Disabling this for now until someone can investigate the hang and/or why timedexec cannot kill the test when it does hang.

Tested on mac (does not skip) and cygwin (does skip).