Merge multiple in download artifact

fix: flatten downloaded artifacts to resolve directory upload error

GitHub Actions downloads artifacts into directories named after the artifact.
This caused the release workflow to try uploading directories instead of files,
resulting in 'is a directory' errors. The fix flattens the artifacts by moving
files from their directories to the root before release creation.

Author: rezzmah

.github/workflows/release.yml

@@ -143,6 +143,9 @@ jobs:
           fetch-depth: 0
       - name: Download artifacts
         uses: actions/download-artifact@v4
+        with:
+          pattern: bera-reth-*
+          merge-multiple: true
       - name: Generate full changelog
         id: changelog
         run: |