sg: fix release uploads

Created by: bobheadxi

Fixes the issues with releasing to a stable name in https://github.com/sourcegraph/sourcegraph/pull/25074#discussion_r712843319

• Run: https://github.com/sourcegraph/sourcegraph/actions/runs/1266241687
• Release: https://github.com/sourcegraph/sg/releases/tag/2021-09-23-14-32-483967d9