nixos/modules: Split recursive mkdir with permissions

      ''

        # Create the required /bin/sh symlink; otherwise lots of things

        # (notably the system() function) won't work.

        mkdir -m 0755 -p /bin

        mkdir -p /bin

        chmod 0755 /bin

        ln -sfn "${cfg.binsh}" /bin/.sh.tmp

        mv /bin/.sh.tmp /bin/sh # atomically replace /bin/sh

      '';