wic: use shutil.which

Use shutil.which to find the executable instead to silence the below warning:
 $ cat tmp/work/intel_x86_64-poky-linux/core-image-base/1.0-r5/temp/log.do_image_wic
 [snip]
 DEBUG: Executing shell function do_image_wic
 /path/layers/oe-core/scripts/wic:27: DeprecationWarning: The distutils package is deprecated and slated for removal in Python 3.12. Use setuptools or check PEP 632 for potential alternatives
  from distutils import spawn
 INFO: Creating image(s)...
 [snip]

[RP: Added conversion for missed function reference]
Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3966cbf5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>