When the nixpkgs-check-by-name tooling is updated, it should be run on Nixpkgs in the same PR. This also relates to #256788.

This could be achieved using passthru.tests on the tests.nixpkgs-check-by-name derivation which copies Nixpkgs to the store and then runs the tooling on it in a derivation. This would then automatically be run when a commit with the tests.nixpkgs-check-by-name: prefix is in the PR. This is however fairly slow and expensive due to the Nixpkgs copy.