Added branch reference in test-nightly-unstable

Author: dariaguy

.github/workflows/test-nightly-unstable.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   build-and-test-unstable:
-    uses: redis/docker-library-redis/.github/workflows/pre-merge.yml
+    uses: redis/docker-library-redis/.github/workflows/pre-merge.yml@unstable
     with:
       release_tag: unstable
       redisearch_version: master