Make release creation idempotent (lookup existing release by tag first)

scripts/release-helper.py

@@ -71,6 +71,8 @@ def main() -> int:
         return extract_field("id")
     if cmd == "extract-upload-url":
         return extract_field("upload_url")
+    if cmd == "extract-tag":
+        return extract_field("tag_name")
     return usage()