Fix downloadCached() with a chroot store
Fix downloadCached() with a chroot store

E.g.

  nix run --store ~/my-nix -f channel:nixos-17.03 hello -c hello

This problem was mentioned in #1897.

vcunat/nix,pikajude/nix,shlevy/nix,peti/nix,pikajude/nix,dezgeg/nix,layus/nix,zimbatm/nix,vcunat/nix,zimbatm/nix,NixOS/nix,peti/nix,ehmry/nix,shlevy/nix,zimbatm/nix,peti/nix,ehmry/nix,rycee/nix,ehmry/nix,layus/nix,ehmry/nix,layus/nix,rycee/nix,dezgeg/nix,layus/nix,dezgeg/nix,layus/nix,dezgeg/nix,rycee/nix,ehmry/nix,peti/nix,rycee/nix,vcunat/nix,vcunat/nix,pikajude/nix,dezgeg/nix,zimbatm/nix,NixOS/nix,shlevy/nix,dtzWill/nix,NixOS/nix,peti/nix,NixOS/nix,rycee/nix,dtzWill/nix,ehmry/nix,dtzWill/nix,dtzWill/nix,shlevy/nix,NixOS/nix,dtzWill/nix,shlevy/nix