Don't run preSwitchChecks during install

NixOS includes an option called `system.preSwitchChecks` since
NixOS/nixpkgs@6e192c4.

As we are installing a new system here, we do not care too much about the systems state pre-switching and therefore disable those checks. 

This change is motivated by nix-community/srvos#591 (comment)

Author: phaer

src/nixos-anywhere.sh

@@ -614,7 +614,7 @@ if [ ${copyHostKeys-n} = "y" ]; then
     cp -a "\$p" "/mnt/\$p"
   done
 fi
-nixos-install --no-root-passwd --no-channel-copy --system "$nixosSystem"
+NIXOS_NO_CHECK=1 nixos-install --no-root-passwd --no-channel-copy --system "$nixosSystem"
 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