fix(ci): use RUNNER_TEMP for all temp files instead of /tmp

/tmp on the self-hosted runner has sticky bit — files created by
previous runs (or different users) can't be overwritten. RUNNER_TEMP
is per-run and owned by the runner user.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/infrastructure/scripts/prepare-auto-iso.sh

@@ -28,12 +28,12 @@ echo "  Answer file:  $ANSWER_FILE"
 echo "  First boot:   ${FIRST_BOOT:-none}"
 echo "  Output:       $OUTPUT_ISO"
 
-# Use /tmp for staging to avoid permission issues with the source ISO directory
+# Use the output directory for staging to avoid permission issues with the source ISO directory
 proxmox-auto-install-assistant prepare-iso \
     --fetch-from iso \
     --answer-file "$ANSWER_FILE" \
     "${FIRST_BOOT_ARGS[@]}" \
-    --tmp /tmp \
+    --tmp "$(dirname "$OUTPUT_ISO")" \
     --output "$OUTPUT_ISO" \
     "$BASE_ISO"