Allow skipping system.preSwitchChecks

As they might be written for switching from one generation to another,
without initial installation in mind and failing checks would prevent
successful installation.

Note that the first line is just string substition, as SSH doesn't
pass through arbitrary env variables as that would have far-reaching
security implications.

Co-Authored-By: Michael Hoang <enzime@users.noreply.github.com>

Author: phaer

src/nixos-anywhere.sh

@@ -719,6 +719,10 @@ if [ ${copyHostKeys-n} = "y" ]; then
     cp -a "\$p" "/mnt/\$p"
   done
 fi
+# https://stackoverflow.com/a/13864829
+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 command -v zpool >/dev/null && [ "\$(zpool list)" != "no pools available" ]; then