Avoid copying nixpkgs to the store a second time in nix-shell.

Without this, NIX_PATH points at a copy of nixpkgs in /nix/store.

Specifically, before this patch, NIX_PATH was

/nix/store/rq6cnxd23wzdhvq2a1jj53n5bbrx8yqy-vv70nkylllk1qrvfhl0153r2iy1dfxs9-release2105

and after this patch it is

/nix/store/vv70nkylllk1qrvfhl0153r2iy1dfxs9-release2105

This is a small cleanup I noticed while working privatestorageops#382.