ci: publish: use 'aws s3 cp' to upload artifacts

'aws s3 sync' expects directories as arguments, but we are uploading a
single file, so use 'aws s3 cp' instead. Also make sure the path ends
with a slash for protection rules to apply.

Author: pillo79

.github/workflows/package_core.yml

@@ -179,7 +179,7 @@ jobs:
           aws-region: ${{ secrets.AWS_REGION }}
 
       - name: Upload artifact
-        run: aws s3 sync ${{ env.CORE_ARTIFACT }}.tar.bz2 s3://${{ secrets.S3_BUCKET }}
+        run: aws s3 cp ${{ env.CORE_ARTIFACT }}.tar.bz2 s3://${{ secrets.S3_BUCKET }}/
 
       - name: Prepare package index snippet
         run: ./extra/gen_package_index_json.sh