Again an issue with the pinned nixpkgs vs NIX_PATH

This prevents some weird-looking ordering issues when the ratman-webui
assets are built as a side effect of the ratman targets.

There's an upstream nixpkgs issue, which breaks eg. `pkgsStatic.yarn2nix`:
https://github.com/NixOS/nixpkgs/issues/116207

Thankfully, the output of `ratman-webui` is a pile of arch-independent,
static assets, so we don't actually need this to work; we only need the
build scripts to run on the builder.

--------------------------------------------------------------------------

> $ nix-shell -p pkgsStatic.yarn2nix
> error: attribute 'runtimeShell' missing
>
>        at /home/embr/src/nixpkgs/pkgs/top-level/all-packages.nix:731:17:
>
>           730|       substitutions = {
>           731|         shell = targetPackages.runtimeShell;
>              |                 ^
>           732|         passthru.tests = tests.makeWrapper;
> (use '--show-trace' to show detailed location information)

This embeds the built Ember.js Web UI into the binary (release), or reads
them from the source tree (development), which means we don't have to worry
about where on disk the assets are relative to the binary.

But it comes with the caveat that building `ratmand` with `--feature=webui`
will now error unless you've previously ran `cd ratman/webui; yarn build`.

This is already a boolean in the API response; adding a transformation has
a slight runtime performance penalty.