fix(ci): avoid unbound module variable in purity check

Keep module expansion inside the container shell when probing Python imports, preventing set -u from failing in the host script.

scripts/verify-deployable-image-purity.sh

@@ -157,7 +157,7 @@ if [ -z "$PYTHON_BIN" ]; then
 fi
 
 for module in ${MODULES}; do
-  if "$PYTHON_BIN" -c "import importlib.util,sys; sys.exit(0 if importlib.util.find_spec('${module}') else 1)" >/dev/null 2>&1; then
+  if "$PYTHON_BIN" -c "import importlib.util,sys; sys.exit(0 if importlib.util.find_spec(\"$module\") else 1)" >/dev/null 2>&1; then
     printf "%s\n" "$module"
   fi
 done