Simplify logic for maintainer approvals

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There's no need to loop through lists of approved users and maintainers to find common users, when one can simply do a set intersection if they are constructed as sets.

By simplifying the logic as above, one also avoids adding the label 12.approved-by: package-maintainer twice when two maintainers have given their approval.

[wegank]

Tested on the last 1000 PRs in Nixpkgs.