The docker-images Gentoo job can't install libimagequant at the moment - https://github.com/python-pillow/docker-images/actions/runs/10988523287/job/30505070454#step:6:1356

cp: cannot stat 'usr/lib/libimagequant.so*': No such file or directory

Testing, I find that the shared libraries are actually within 'lib64' instead. So this updates the install script to work with either directory.

With this change, libimagequant is installed and detected successfully.