Redirect FETCH_PBUILDER_DIRECTORY in release.sh

Change-Id: Ia18f8d93e7ab71893e0bb7a82b4c582bb08fc4b0
diff --git a/kokoro/release.sh b/kokoro/release.sh
index 4e8daa9..11fb227 100644
--- a/kokoro/release.sh
+++ b/kokoro/release.sh
@@ -14,6 +14,7 @@
 export TARBALL_FETCH_ROOT_DIRECTORY=${KOKORO_GFILE_DIR}
 export PREBUILT_MODULES_ROOT=${KOKORO_GFILE_DIR}
 export PREBUILT_DOCKER_ROOT=${KOKORO_GFILE_DIR}
+export FETCH_PBUILDER_DIRECTORY=${KOKORO_GFILE_DIR}
 export DEBOOTSTRAP_TARBALL_REVISION=.
 export ROOTFS_REVISION=.