Do not flag draft PRs with `needs-approved-design` label

[bzp99]

I think it’s enough to add these to non-draft PRs

[Sreejesh06]

Maybe we could skip draft PRs in the check since drafts are still WIP, so applying needs-approved-design at that stage may be premature. We could handle this by checking pull_request.draft in check_design_approval.js or adding if: !github.event.pull_request.draft in the workflow.