Fix memtest hook: grub.cfg/live.cfg missing during binary hooks is expected

lb binary_grub-efi and lb binary_syslinux create these files from templates
that already have memtest entries hardcoded. The hook should not fail when
the files don't exist yet — validate_iso_memtest() checks the final ISO.
Only the binary files (x64.bin, x64.efi) are required here.

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

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

@@ -26,6 +26,14 @@ fail_or_warn() {
     return 0
 }
 
+# grub.cfg and live.cfg may not exist yet when binary hooks run — live-build
+# creates them after this hook (lb binary_grub-efi / lb binary_syslinux).
+# The template already has memtest entries hardcoded, so a missing config file
+# here is not an error; validate_iso_memtest() checks the final ISO instead.
+warn_only() {
+    log "WARNING: $1"
+}
+
 copy_memtest_file() {
     src="$1"
     dst_name="${2:-$(basename "$src")}"
@@ -61,15 +69,17 @@ 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"
@@ -133,7 +143,7 @@ ensure_memtest_binaries() {
 
 ensure_grub_entry() {
     [ -f "$GRUB_CFG" ] || {
-        fail_or_warn "missing ${GRUB_CFG}"
+        warn_only "missing ${GRUB_CFG} (will be created by lb binary_grub-efi from template)"
         return 0
     }
 
@@ -159,7 +169,7 @@ EOF
 
 ensure_isolinux_entry() {
     [ -f "$ISOLINUX_CFG" ] || {
-        fail_or_warn "missing ${ISOLINUX_CFG}"
+        warn_only "missing ${ISOLINUX_CFG} (will be created by lb binary_syslinux from template)"
         return 0
     }