fix: set TMPDIR=/var/tmp to avoid tmpfs overflow during mkinitfs

iso/builder/build-debug.sh

@@ -97,8 +97,10 @@ cp "${BUILDER_DIR}/mkimg.bee_debug.sh" "${HOME}/.mkimage/"
 # Export overlay dir so the profile script can find it regardless of SRCDIR.
 export BEE_OVERLAY_DIR="${OVERLAY_DIR}"
 
-# mkimage.sh uses git internally; run from /tmp to avoid conflicts with bee repo.
-cd /tmp
+# Run from /var/tmp to avoid git repo context conflicts and to ensure enough scratch space.
+# mkinitfs/update-kernel use TMPDIR for initramfs build; tmpfs /tmp is only ~1GB.
+export TMPDIR=/var/tmp
+cd /var/tmp
 sh /usr/share/aports/scripts/mkimage.sh \
     --tag "v${ALPINE_VERSION}" \
     --outdir "${DIST_DIR}" \