Use correct variable name to detect removal step

Due to this mistake, the helpers were not called in reverse order
during the removal step. This lead to things like "apt update" failing
because a broken /etc/resolv.conf has been restored before the call
to "chroot_archives remove".

Gbp-Dch: Ignore

scripts/build/chroot_prep

@@ -98,7 +98,7 @@ if In_list "archives" ${HELPERS} && [ ! -e .build/bootstrap ]; then
 	Echo_warning "The 'archives' helper is selected, but the bootstrap stage has not completed."
 fi
 
-if [ "${_ACTION}" = "remove" ]; then
+if [ "${ACTION}" = "remove" ]; then
 	HELPERS_REVERSED=""
 	for HELPER in $HELPERS; do
 		HELPERS_REVERSED="${HELPER}${HELPERS_REVERSED:+ $HELPERS_REVERSED}"