build.cc: fix bind-mount of /dev/{pts,ptmx} fallback
build.cc: fix bind-mount of /dev/{pts,ptmx} fallback

Don't bind-mount these to themselves,
mount them into the chroot directory.

Fixes pty issues when using sandbox on CentOS 7.4.
(build of perlPackages.IOTty fails before this change)

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