workflows/check-by-name.yml: Switch to new separate repo

The nixpkgs-check-by-name tooling is [being moved](https://github.com/NixOS/nixpkgs/issues/286559#issuecomment-2000466124)
to a [separate repo](https://github.com/NixOS/nixpkgs-check-by-name).

This commit updates Nixpkgs CI to use it instead of the tree inside
Nixpkgs

No changes have been made to the tooling locally since it was moved:
- [Exported history](https://github.com/NixOS/nixpkgs/commits/55bf02190ee57fcf83490fd7b6bf7834e28c9c86/pkgs/test/nixpkgs-check-by-name)
- [Imported history](https://github.com/NixOS/nixpkgs-check-by-name/commits/d579e1821d56c79fd90dab34b991cc7bdab7a5c6/)

1 files changed, 31 insertions, 0 deletions

diff --git a/pkgs/test/check-by-name/README.md b/pkgs/test/check-by-name/README.md
new file mode 100644
index 0000000000000..c68e7a93b7d0e
--- /dev/null
+++ b/pkgs/test/check-by-name/README.md
@@ -0,0 +1,31 @@
+# `pkgs/by-name` check CI scripts
+
+This directory contains scripts and files used and related to the CI running the `pkgs/by-name` checks in Nixpkgs.
+See also the [CI GitHub Action](../../../.github/workflows/check-by-name.yml).
+
+## `./run-local.sh BASE_BRANCH [REPOSITORY]`
+
+Runs the `pkgs/by-name` check on the HEAD commit, closely matching what CI does.
+
+Note that this can't do exactly the same as CI,
+because CI needs to rely on GitHub's server-side Git history to compute the mergeability of PRs before the check can be started.
+In turn when running locally, we don't want to have to push commits to test them,
+and we can also rely on the local Git history to do the mergeability check.
+
+Arguments:
+- `BASE_BRANCH`: The base branch to use, e.g. master or release-23.11
+- `REPOSITORY`: The repository to fetch the base branch from, defaults to https://github.com/NixOS/nixpkgs.git
+
+## `./update-pinned-tool.sh`
+
+Updates the pinned [nixpkgs-check-by-name tool](https://github.com/NixOS/nixpkgs-check-by-name) in [`./pinned-version.txt`](./pinned-version.txt) to the latest [release](https://github.com/NixOS/nixpkgs-check-by-name/releases).
+Each release contains a pre-built x86_64-linux version of the tool which is used by CI.
+
+This script currently needs to be called manually when the CI tooling needs to be updated.
+
+Why not just build the tooling right from the PRs Nixpkgs version?
+- Because it allows CI to check all PRs, even if they would break the CI tooling.
+- Because it makes the CI check very fast, since no Nix builds need to be done, even for mass rebuilds.
+- Because it improves security, since we don't have to build potentially untrusted code from PRs.
+  The tool only needs a very minimal Nix evaluation at runtime, which can work with [readonly-mode](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-readonly-mode) and [restrict-eval](https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-restrict-eval).
+