Allow ramdisk-create to run without $USER set

It's possible this is run form an environment where $USER isn't set,
properly fallback to whoami in this case.

Change-Id: I1181f714c3c456ee264b34d282bac5c0adb67a0e

elements/ironic-agent/cleanup.d/99-ramdisk-create

@@ -11,6 +11,8 @@ set -o pipefail
 
 [ -n "$TARGET_ROOT" ]
 
+USER=${USER:-$(whoami)}
+
 source $_LIB/img-functions
 
 IMAGE_PATH=$(readlink -f $IMAGE_NAME)