sshd: fix shellcheck warnings in prestart script (#340484)

                      if ! [ -h "${k.path}" ]; then

                          rm -f "${k.path}"

                      fi

                      mkdir -m 0755 -p "$(dirname '${k.path}')"

                      mkdir -p "$(dirname '${k.path}')"

                      chmod 0755 "$(dirname '${k.path}')"

                      ssh-keygen \

                        -t "${k.type}" \

                        ${lib.optionalString (k ? bits) "-b ${toString k.bits}"} \