Update tarball directory variable in preamble

Change-Id: I4887498cf85cd8f89a2133fda9affaa773833e88
diff --git a/preamble.mk b/preamble.mk
index 1e02f72..cae5c70 100644
--- a/preamble.mk
+++ b/preamble.mk
@@ -88,5 +88,5 @@
 		--exclude=debfoster \
 		--include=$$(echo $(DEBOOTSTRAP_EXTRA) |tr ' ' ',') \
 
-GFS_ROOT_DIRECTORY := \
+TARBALL_FETCH_ROOT_DIRECTORY := \
 	/google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise