[Build] disable DOCKER_BUILDKIT explicitly (#14405)

Why I did it
Fix #14081
By default DOCKER_BUILDKIT is enabled after docker version 23.0.0
So we need to disable it explicitly if SONIC_USE_DOCKER_BUILDKIT is not set.
Otherwise it will produce larger installable images.

How I did it
set DOCKER_BUILDKIT=0 in slave.mk

How to verify it

Author: k-v1

slave.mk

@@ -454,6 +454,8 @@ include Makefile.cache
 ifeq ($(SONIC_USE_DOCKER_BUILDKIT),y)
 $(warning "Using SONIC_USE_DOCKER_BUILDKIT will produce larger installable SONiC image because of a docker bug (more details: https://github.com/moby/moby/issues/38903)")
 export DOCKER_BUILDKIT=1
+else
+export DOCKER_BUILDKIT=0
 endif