Use `shutil.which` instead of `sh.which`

[misl6]

• Recently our CI started to fail when checking the availability of certain tools (E.g. ccache ...) See: https://github.com/kivy/kivy-ios/runs/7413749298
• sh released a new version (1.14.3, 12 hours ago) with a breaking change on our side.
• This is due to https://github.com/amoffat/sh/pull/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.