GNUInstallDirs: Prefer system lib64 over conda lib when ambiguous

Update logic added by commit ecaca8c129 (GNUInstallDirs now aware of
conda lib directory requirements, 2021-09-08, v3.22.0-rc1~142^2).
When it is ambiguous if we are doing a conda install or a system
install prefer using the system library directory.

Fixes: #22962