Move more of the initialization into nix::initLibStore

[roberth]

Is your feature request related to a problem? Please describe.

This describes a tech debt, not necessarily a current problem, hence the dx label. (should we have a tech debt label?)

#7478 creates initLibStore to initialize, well, libstore, but actually most (or all) of the initialization happens in initNix from libmain.

For the purpose of good architecture, libstore-specific initialization should be moved into this new function, and/or into an initialization function in libutil (for things that are common to all nix libraries)

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