The machines_dir() function was hardcoding "machines" without considering
the directory parameter from buildClan/clan configuration. This caused
update-hardware-config and other commands to write files to the wrong
location when a custom directory was specified (e.g., directory = ./clan).
Solution:
1. Added relativeDirectory to inventoryClass in Nix, computed where both
self and directory have consistent store paths during evaluation
2. Updated machines_dir() to use this pre-computed relative path from Nix
via flake.select("clanInternals.inventoryClass.relativeDirectory")
Fixes: https://git.clan.lol/clan/clan-core/issues/2906
