collab: Look up users using github_user_id when backfilling (#16708)

This PR updates the user backfiller to look up the GitHub users using
the `github_user_id` instead of `github_login`.

Release Notes:

- N/A

crates/collab/src/user_backfiller.rs

@@ -75,8 +75,8 @@ impl UserBackfiller {
         for user in users_missing_github_user_created_at {
             match self
                 .fetch_github_user(&format!(
-                    "https://api.github.com/users/{}",
-                    user.github_login
+                    "https://api.github.com/user/{}",
+                    user.github_user_id
                 ))
                 .await
             {