zfsbootmenu: make 'less' an optional binary

'less' is only used by debug logging viewer, so there's no need to
refuse to build an image if the host system doesn't have that binary
available.

zfsbootmenu/install-helpers.sh

@@ -34,7 +34,6 @@ zfsbootmenu_essential_binaries=(
   "lsmod"
   "depmod"
   "dmesg"
-  "less"
   "fzf"
   "setsid"
   "cat"
@@ -44,6 +43,7 @@ zfsbootmenu_essential_binaries=(
 zfsbootmenu_optional_binaries=(
   "mbuffer"
   "column"
+  "less"
 )
 
 # shellcheck disable=SC2034