oiiotool: use a shared IC (#2287)
oiiotool: use a shared IC (#2287)

I can't think of any reason why the old code was making a custom
cache, but it interfered with setting certain cache options from
the command line.
OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio