ci: update actions/download-artifact to v4

This version bump is now needed because of changes from commit
7e2c233674, which ended up breaking
automated uploads to the Releases page.

.github/workflows/ci.yml

@@ -210,7 +210,7 @@ jobs:
           echo "date_now=$(date --rfc-3339=seconds)" >> "${GITHUB_OUTPUT}"
 
       - name: Download build artifacts from previous jobs
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           path: artifacts