GitHub Actions: Run mixed-version testing against 4.0.3 explicitly

... not any release.

[Why]
Once we release RabbitMQ 4.1.0, it means that the `v4.1.x` branch would
be tested against 4.1.x, i.e. itself.

Note that we will have to update this pinned version from time to time
if needed.

Author: dumbbell

.github/workflows/test-make-target.yaml

@@ -57,8 +57,9 @@ jobs:
       uses: dsaltares/fetch-gh-release-asset@master
       if: inputs.mixed_clusters
       with:
+        version: 'tags/v4.0.3'
         regex: true
-        file: "rabbitmq-server-generic-unix-[\\d.]*\\.tar.xz"
+        file: "rabbitmq-server-generic-unix-[\\d.]*\\.tar\\.xz"
         target: ./
 
     - name: MIXED CLUSTERS - SETUP SECONDARY_DIST