[ci] Fix false reference in docker slave build pipeline. sonic-net#19188

Why I did it
When upgrade to bookworm container, this file is changed by mistake.
bookworm branch is out of date. It breaks pipeline to build sonic-slave-*.

Work item tracking
Microsoft ADO (number only): 28290170
How I did it
How to verify it

Author: liushilongbuaa

.azure-pipelines/docker-sonic-slave.yml

@@ -8,7 +8,7 @@ resources:
   - repository: buildimage
     type: github
     name: sonic-net/sonic-buildimage
-    ref: bookworm
+    ref: master
     endpoint: sonic-net
 
 schedules: