use sources.list to calc sonic-slave-user tag (#14409)

Why I did it
This is a fix for c63e9fe
SLAVE_TAG should include all dependencies used for SLAVE_BASE_TAG

How I did it
Take sources.list.* into account when calculate SLAVE_TAG

How to verify it

Makefile.work

@@ -245,6 +245,7 @@ SLAVE_BASE_TAG = $(shell \
 SLAVE_TAG = $(shell \
 	(cat $(SLAVE_DIR)/Dockerfile.user \
 		 $(SLAVE_DIR)/Dockerfile \
+		 $(SLAVE_DIR)/sources.list.* \
 		 $(SLAVE_DIR)/buildinfo/versions/versions-* \
 	&& echo $(USER)/$(PWD)/$(CONFIGURED_PLATFORM)) \
 	| sha1sum \