fix: find Nix store dir with `builtins.storeDir`

[tomeon]

rather than attempting to do so with builtins.storePath, which is for "defin[ing] a dependency on an already existing store path" (https://nixos.org/manual/nix/stable/language/builtins.html#builtins-storePath).

[zimbatm]

thanks!

bors merge

[bors[bot]]

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