workflows/check-by-name: Pin nixpkgs-check-by-name tool

Before this, the tool for CI would update when nixos-unstable updated,
which is kind of terrible because you don't know when it happens, and it
might break master.

In fact, the tooling _right now_ has a serious bug and shouldn't be used!

This PR addresses this by _pinning_ the tooling in Nixpkgs itself.

Updating the tooling now requires two PRs:
- The first PR to update the tooling source
- (wait for Hydra to build and publish it in nixos-unstable)
- The second PR to update the pinned tooling

In turn you know exactly when the changes are going to take effect.

This change however has additional benefits:
- It makes CI more reproducible, because it doesn't depend on the state
  of nixos-unstable anymore
- Updates to the tooling can be tested with the workflow itself,
  because PRs that update the pinned tool will be tested on the updated
  version
- CI gets a sizable speed boost, because there's no need to download and
  evaluate a channel anymore
- It makes it more realistic to move the source of the tool into a
  separate repository
- It removes the brittle branch-specific logic that was previously
  needed to ensure that release branches use their own version of the
  tooling.

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