fix IMAGE_NAME for mirror image

Change-Id: I7eb611632bd880d58d8e409dba37e493b865e3fd
diff --git a/build-mirror-image.groovy b/build-mirror-image.groovy
index 4bab086..4c42b3e 100644
--- a/build-mirror-image.groovy
+++ b/build-mirror-image.groovy
@@ -72,6 +72,7 @@
                                 "PACKER_LOG_PATH=${workspace}/packer.log",
                                 "PACKER_LOG=1",
                                 "TMPDIR=${workspace}/tmp",
+                                "IMAGE_NAME=${IMAGE_NAME}",
                                 "OS_USERNAME=${creds.username}",
                                 "OS_PASSWORD=${creds.password.toString()}"]) {
             if (PACKER_DEBUG.toBoolean()) {