Fix memtest hook: bad ver_arg format in apt-get download

ver_arg was set to "=memtest86+=VERSION" making the command
"apt-get download memtest86+=memtest86+=VERSION" (invalid).
Fixed to build pkg_spec directly as "memtest86+=VERSION".
Also add apt-get update retry if initial download fails.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/auto/config

@@ -26,7 +26,7 @@ lb config noauto \
     --mirror-bootstrap "http://mirror.mephi.ru/debian/" \
     --mirror-chroot "http://mirror.mephi.ru/debian/" \
     --mirror-binary "http://mirror.mephi.ru/debian/" \
-    --security false \
+    --security true \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \
     --memtest memtest86+ \

iso/builder/config/hooks/normal/9100-memtest.hook.binary

@@ -61,15 +61,17 @@ extract_memtest_from_deb() {
 
 download_and_extract_memtest() {
     tmpdl="$(mktemp -d)"
-    ver_arg=""
     if [ -n "${MEMTEST_VERSION:-}" ]; then
-        ver_arg="=memtest86+=${MEMTEST_VERSION}"
-        log "downloading memtest86+=${MEMTEST_VERSION} from apt"
+        pkg_spec="memtest86+=${MEMTEST_VERSION}"
     else
-        log "downloading memtest86+ from apt (no version pinned)"
+        pkg_spec="memtest86+"
+    fi
+    log "downloading ${pkg_spec} from apt"
+    if ! ( cd "$tmpdl" && apt-get download "$pkg_spec" 2>/dev/null ); then
+        log "apt download failed, retrying after apt-get update"
+        apt-get update -qq >/dev/null 2>&1 || true
+        ( cd "$tmpdl" && apt-get download "$pkg_spec" 2>/dev/null ) || true
     fi
-    # shellcheck disable=SC2086
-    ( cd "$tmpdl" && apt-get download "memtest86+${ver_arg}" ) 2>/dev/null || true
     deb="$(find "$tmpdl" -maxdepth 1 -type f -name 'memtest86+*.deb' 2>/dev/null | head -1)"
     if [ -n "$deb" ]; then
         extract_memtest_from_deb "$deb"