workflows/nixpkgs-vet: use nixpkgs-vet from pinned nixpkgs

We have added nixpkgs-vet as a regular package to nixpkgs a while ago,
so we can now use it from pinned nixpkgs. This avoids pulling a
platform-specific binary version from upstream.

This change also allows to run the tool easily locally, the same way as
other tools:

  nix-build ci -A nixpkgs-vet

This will do a full check of the repo with the exception of
nixpkgs-vet's "ratchet" checks: Those depend on having two branches to
compare, but the default is to only look at the head branch. Those
ratchet checks will still be run in CI, though.

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