A CACHEDIR.TAG file indicates that the contents can be automatically
re-generated. This is not really true for Nix store paths. (Well _Nix_
can recreate them, but that's different.)
I noticed this issue as I was restoring full system backup that "for
some reason" always missed /nix/store/*-fc-cache (found by `nix-store
--verify --repair`). Turns out I was excluding caches from my backup...
