Fixes
```
GEN /home/eelco/Dev/nix-master/outputs/out/share/doc/nix/manual/index.html
error: File not found: ../store/types/
┌─ release-notes/rl-next.md:60:197
│
60 │ The build hook protocol did in principle support custom ways of remote building, but that can also be accomplished with a custom service for the ssh or daemon/ssh-ng protocols, or with a custom [store type](../store/types/) i.e. `Store` subclass. <!-- we normally don't mention classes, but consider that this release note is about a library use case -->
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File not found: ../store/types/
Error: One or more incorrect links
2024-08-19 16:47:57 [ERROR] (mdbook::renderer): Renderer exited with non-zero return code.
2024-08-19 16:47:57 [ERROR] (mdbook::utils): Error: Rendering failed
2024-08-19 16:47:57 [ERROR] (mdbook::utils): Caused By: The "linkcheck" renderer failed
```
This is because with the split packages of the Meson build, we simply
have no idea what directory the binaries will be installed in when we
build the library.
In the process of doing so, consolidate and make more sophisticated the
logic to cope with a few corner cases (e.g. `NIX_BIN_DIR` exists, but no
binaries are inside it).
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
