hwmonN sensor paths can change

[Alia5]

As seen in https://github.com/NGnius/Fantastic/blob/main/backend-rs/src/sys.rs, the paths to the sensors are hardcoded to .../hwmon5/...

This really isn't ideal as hwmon indices aren't fixed but dependent on module loading order and thus may change on reboot.
(For me personally fan1_input isn in hwmon4 so I cannot really use this Fantastic (pun intended) plugin)

To fix this, the index should be queried dynamically instead of being hardcoded

[NGnius]

From my experience, module load order is consistent on Steam Deck so this is good enough. If you're using a different kernel or a completely different system then it's going to break but then that's fine since Fantastic only supports SteamOS 3 on Steam Deck right now.