v3: GCS metadata acl key should be within a list

While testing GCS within v3 file permissions were not set to public on deployment. After comparing the metadata to the documentation, the `acl` key should be a list of permissions according to the docs (https://developers.google.com/resources/api-libraries/documentation/storage/v1beta2/python/latest/storage_v1beta2.objects.html#insert). With this change permissions are set successfully on deployment.