fix: set PR labels serially

[lukekarrys]

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Fixes #<issue_number_goes_here> 🦕

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I did not open an issue for this first. But the old behavior of using Promise.all to set labels was causing GitHub API errors for us on https://github.com/npm/cli in CI. I've lost the logs for CI runs when this was happening but I've been using this fix in our fork (which we wan't to get off of, hence the PR) as it fixed the issue.