stage-1: fix copytoram option for loopback.cfg boot

Uses an alternative method to find the size of the ISO when it is a file and
not a block device for creating the tmpfs its contents are copied to.

nixos/modules/system/boot/stage-1-init.sh

@@ -542,7 +542,7 @@ while read -u 3 mountPoint; do
     # If copytoram is enabled: skip mounting the ISO and copy its content to a tmpfs.
     if [ -n "$copytoram" ] && [ "$device" = /dev/root ] && [ "$mountPoint" = /iso ]; then
       fsType=$(blkid -o value -s TYPE "$device")
-      fsSize=$(blockdev --getsize64 "$device")
+      fsSize=$(blockdev --getsize64 "$device" || stat -Lc '%s' "$device")
 
       mkdir -p /tmp-iso
       mount -t "$fsType" /dev/root /tmp-iso