Update download artifact step

Now that we run the build skia step outside the main branch, we need to download only the successful artifacts from the main branch. Running the build-skia outside the main branch, won't upload any artifacts.

Author: wcandillon

.github/workflows/android-ubuntu.yml

@@ -43,6 +43,7 @@ jobs:
           workflow: "build-skia.yml"
           repo: shopify/react-native-skia
           path: artifacts
+          branch: main
 
       - name: Copy Artifacts to libs folder
         run: yarn workflow-copy-libs