nix run: Fix accidental removal of /nix/store existence check
nix run: Fix accidental removal of /nix/store existence check

Parenthetical to #1686, we don't need to create a new root if we can
just bind-mount on top of the existing /nix/store.

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