Update trampoline.sh (#302)

Clean up temp files at exit.

.kokoro/trampoline.sh

@@ -1,5 +1,4 @@
 #!/bin/bash
-
 # Copyright 2017 Google Inc.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -14,4 +13,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+set -eo pipefail
+
+# Always run the cleanup script, regardless of the success of bouncing into
+# the container.
+
+function cleanup() {
+    chmod +x ${KOKORO_GFILE_DIR}/trampoline_cleanup.sh
+    ${KOKORO_GFILE_DIR}/trampoline_cleanup.sh
+    echo "cleanup";
+}
+trap cleanup EXIT
+
 python3 "${KOKORO_GFILE_DIR}/trampoline_v1.py"