build: merge multiple on download artifact

When there are multiple artifacts, `actions/download-artifact` creates
subdirectories for each artifact, and results in incorrect folder
structure assumptions in later steps.

Signed-off-by: Chengzhong Wu <cwu631@bloomberg.net>

Author: legendecas

.github/workflows/build-shared.yml

@@ -39,6 +39,8 @@ jobs:
       - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c  # v8.0.1
         with:
           path: tarballs
+          pattern: '*.tar.gz'
+          merge-multiple: true
 
       - name: Extract tarball
         shell: bash