CI: use the version of libxml2 and zlib located at https://github.com/cmlibs-dependencies

[agarny]

https://github.com/cellml/gha contains prebuilt binaries for libxml2 and zlib for use in GitHub Actions. However, those binaries come from https://github.com/OpenCMISS-Dependencies/libxml2/releases/tag/v2.9.10-1 and https://github.com/OpenCMISS-Dependencies/zlib/releases/tag/v1.2.3-1, respectively, while we should now be building libxml2 and zlib from https://github.com/cmlibs-dependencies/libxml2/tree/v2.9.10 and https://github.com/cmlibs-dependencies/zlib/tree/v1.2.12, respectively.