CI: use merge-multiple to ensure downloaded artifacts are in one dir

(cherry picked from commit a8afeac)

Author: rgommers

.github/workflows/wheels.yml

@@ -235,6 +235,7 @@ jobs:
         uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5.0.0
         with:
           path: dist
+          merge-multiple: true
 
       - name: Remove wasm wheels
         run: |
@@ -296,6 +297,7 @@ jobs:
         uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5.0.0
         with:
           path: dist
+          merge-multiple: true
 
       - name: Remove wasm wheels
         run: |