GNUInstallDirs: Prefer system lib64 over conda lib when ambiguous

Update logic added by commit ecaca8c1 (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
39 jobs for !6773 with fix_fedora_conda_interaction in 33 minutes and 27 seconds (queued for 4 seconds)
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