The local `scratch` override lives under `corgi-patches`, but the Nix
build source filter omitted that directory. As a result, `nix develop`
failed while loading the patched dependency.

Include `corgi-patches` in the filtered source and preserve the patch
implementation in Crane's dummy dependency source. The latter is
required because `cxx-build` calls `scratch::path` while dependency
artifacts are built.

## Testing

- `nix fmt -- --ci nix/build.nix`
- `nix develop --command true`
- `nix develop --command cargo fmt --check`

Release Notes:

- N/A