Flake registries and include paths are not the same thing.
`<nixpkgs>` looks for nixpkgs on the include path, set by `NIX_PATH` and
`--include`. However `--inputs-from` adds inputs to the flake registry,
not the include path.
This changes how we think about this directory; it does not need to be
exclusively for scripts related to updates, but should be a place for
any scripts intended to be run by CI workflows.
This mindset should make it easier to develop and test the business
logic of workflows, without always needing to test "in production" on
the nixvim repo or a fork.
