Clipboard check always looks for xclip

[Dave Shoreman]

If you run nextshot -p to paste from clipboard, it calls check_clipboard to check there's an image to paste.

This function relies on is_wayland which is only set in parse_environment after options are parsed. This is a problem here because check_clipboard is called during arg-parsing, thus NEXTSHOT_ENV isn't set yet (if autodetection is being relied on).