ci: fix artifact download in test-convert job

Update artifact download configuration to use correct artifact name:
- Change from rpm-packages-x86_64 to build-output-x86_64
- Update download-artifact action from v4 to v8 for consistency
- Add merge-multiple: false to prevent unwanted merging behavior

Signed-off-by: Kun Lai <laikun@linux.alibaba.com>

Author: imlk0

.github/workflows/build-rpm.yml

@@ -236,10 +236,11 @@ jobs:
           "
 
       - name: Download artifacts
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
-          name: rpm-packages-x86_64
+          name: build-output-x86_64
           path: ./rpm-packages/
+          merge-multiple: false
 
       - name: Install RPM packages
         run: |