Only use cache.nixos.org when the store is /nix/store
Only use cache.nixos.org when the store is /nix/store

This is consistent with the behaviour of the old
download-from-binary-cache substituter.

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