fix: usr: Disable own memory context for libxml2 on macOS

Apple broke custom memory allocation functions in the system-wide libxml2 starting with macOS Sequoia 15.4.  Usage of the custom memory allocation functions has been disabled on macOS.

Closes #5268

Merge branch '5268-disable-libxml2-memory-management-on-macos' into 'main'

See merge request isc-projects/bind9!10374