Parallels runner only works for *nix based os systems

I did some tests with new Parallels 11 with gitlab runner. The command in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/executors/parallels/executor_parallels.go#L262 expects sudo and ntpdate to be available.

If you're using Windows with cygwin there's no sudo nor ntpdate. Windows user may sync their time by using this in script section of .gitlab-ci.yml:

before_script:
  - w32tm /stripchart /computer:at.pool.ntp.org /dataonly /samples:1

An option to disable this command would be fine, because after commenting out this works.