GitHub Actions: Derive secondary dist version from archive filename

[Why]
Using the `fetch-gh-release-asset` action outputs work fine for our
official releases, but it does not for our alphas published to
`rabbitmq/server-packages` because the filenames use another version
compared to the GitHub release.

[How]
Always derive the version from the archive filename. This is safe
regardless of what the GitHub release states.

Author: dumbbell

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

@@ -65,11 +65,16 @@ jobs:
     - name: MIXED CLUSTERS - SETUP SECONDARY_DIST
       if: inputs.mixed_clusters
       run: |
+        ls -l rabbitmq-server-generic-unix-*.tar.xz*
+
+        archive_name=$(echo rabbitmq-server-generic-unix-*.tar.xz)
+        archive_version=$(echo $archive_name | sed -E -e 's/^rabbitmq-server-generic-unix-//' -e 's/\.tar\.xz$//')
+
         gpg --import rabbitmq-release-signing-key.asc
-        gpg --verify rabbitmq-server-generic-unix-*.asc rabbitmq-server-generic-unix-*.tar.xz
-        tar xf rabbitmq-server-generic-unix-*.tar.xz
+        gpg --verify $archive_name.asc $archive_name
+        tar xf $archive_name
 
-        echo "SECONDARY_DIST=${GITHUB_WORKSPACE}/rabbitmq_server-`echo -n ${{ steps.fetch_secondary_dist.outputs.version }} | sed s/v//`" >> $GITHUB_ENV
+        echo "SECONDARY_DIST=${GITHUB_WORKSPACE}/rabbitmq_server-$archive_version" >> $GITHUB_ENV
 
     - name: SETUP DOTNET (rabbit)
       uses: actions/setup-dotnet@v4