RED-160142 Fix unstable workflow reference (#460)

Author: valeria-raif

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

@@ -7,6 +7,5 @@ on:
 
 jobs:
   build-and-test-unstable:
-    uses: ./.github/workflows/pre-merge.yml@unstable
-    with:
-      ref: 'unstable'
+    uses: redis/docker-library-redis/.github/workflows/pre-merge.yml@unstable
+