Follow-up https://github.com/zed-industries/zed/pull/51876

Use `pull_request_target` for PR size check so the workflow has write
permissions to manage labels on fork PRs. Safe since we don't execute
any PR code here.

Failing example:
https://github.com/zed-industries/zed/actions/runs/23282504031/job/67698793729?pr=48845

Release Notes:

- N/A