update-scripts: move out of flake

See the explanation in the new update-scripts/README.md file.

update-scripts/README.md

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+# Update scripts
+
+This directory contains update scripts that are not part of the actual flake.
+
+These scripts are used by the `update` CI workflow, so you shouldn't need to run them manually.
+This workflow is found at `.github/workflows/update.yml`.
+
+## Developing
+
+Because these scripts aren't packaged in the flake, you should use `nix-build` and `nix-shell` instead of `nix build`, `nix run`, and `nix develop`, etc.
+
+For example, `nix-build -A generate` will build `./generate.nix` into `./result/bin/generate`.
+
+A `shell.nix` is available that will place `generate` on your PATH.
+
+You could use this directory's shell/packages from another working directory by supplying `nix-build` or `nix-shell` with a path.
+E.g. `nix-shell ./update-scripts`.
+
+## Explanation
+
+These packages are not in the flake outputs for two main reasons:
+- Packages built using the flake must follow the flake's `nixConfig`
+- Packages included in the flake's output are checked by `nix flake check`
+
+Being unable to bypass `nixConfig` is an issue because we want to disable [IFD] for the flake, but not for these generate scripts.
+
+If something changes upstream that causes the builds to fail, we don't want this to block us updating `flake.lock`.
+We'd still be made aware of any issues by the `update` CI workflow failing.
+
+[IFD]: https://nixos.org/manual/nix/stable/language/import-from-derivation