nix-shell: Only download requested outputs
nix-shell: Only download requested outputs

Fixes #2116.

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