Use shutil.which instead of sh.which

[misl6]

• Recently our CI started to fail when checking the availability of certain tools (E.g. lld, ccache ...)
  See: https://github.com/kivy/python-for-android/runs/7394641730

• sh released a new version (1.14.3, 12 hours ago) with a breaking change on our side.

• This is due to Make sh.which() an internal function amoffat/sh#584

• They are right, and in order to achieve what we expect shutil.which("name") should be used instead, as returns None when a cmd is not found.

  From Python docs:

  Return the path to an executable which would be run if the given cmd was called. If no cmd would be called, return None.