fix: use --set N:VAR regexp idiom in hiperiso_boot for portability

The earlier hiperiso_boot function used:

  if regexp '.*/([^/]+)' "$iso_path" ; then
      set hiso_base="$1"
  fi

This relies on GRUB's positional $1..$9 capture groups being
visible inside the if-branch. While it parses, the modsrc's own
grub.cfg uses the explicit --set N:VAR form everywhere (e.g. the
FuryBSD/FreeNAS/TrueNAS regexp blocks); switching to that idiom
keeps hiperiso_boot consistent with the rest of the file and avoids
a class of bugs that would only surface on certain GRUB versions
where positional captures interact differently with control flow.

src/grub2/grub/grub.cfg

@@ -2534,12 +2534,13 @@ function hiperiso_boot {
     fi
 
     # Derive log_dir from ISO basename. GRUB script has no ${var##*/}
-    # bash pattern; replicate it with regexp: match ".*/" greedily then
-    # the final segment. If iso_path has no '/', hiso_base stays as the
-    # whole path (acceptable fallback).
+    # bash pattern; replicate it with regexp. Use --set N:VAR syntax
+    # (the same form the Ventoy modsrc uses elsewhere in this file) so
+    # the captured group is bound to a named variable in the same scope
+    # rather than to the positional $1 in the if-statement branch.
     set hiso_base="$iso_path"
-    if regexp '.*/([^/]+)' "$iso_path" ; then
-        set hiso_base="$1"
+    if regexp --set 1:hiso_tail '.*/([^/]+)$' "$iso_path" ; then
+        set hiso_base="$hiso_tail"
     fi
     set hiso_log_dir="/hiperiso/logs/${hiso_base}/"