nixos-anywhere: make install script sh compatible

This actually doesn't matter on NixOS. However I had to install NixOS
from an ubuntu, by faking a NixOS installer, so this was quite handy.

Author: Mic92

src/nixos-anywhere.sh

@@ -745,7 +745,7 @@ if [ ! -z ${NIXOS_NO_CHECK+0} ]; then
   export NIXOS_NO_CHECK
 fi
 nixos-install --no-root-passwd --no-channel-copy --system "$nixosSystem"
-if [[ ${phases[reboot]} == 1 ]]; then
+if [ ${phases[reboot]} == 1 ]; then
   if command -v zpool >/dev/null && [ "\$(zpool list)" != "no pools available" ]; then
     # we always want to export the zfs pools so people can boot from it without force import
     umount -Rv /mnt/