lib/package_manager: Use shutil.copy instead of bb.utils.copyfile for intercepts

If the scripts/postinst-intercepts is owned by root/root then the copyfile() calls
will fail due to chown issues. We don't care about ownership of these files so
use shutil.copy() instead which won't perform any chown.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>