nix: Remove (re)definition of KBUILD_OUTPUT from package.nix

[delet-this]

Resolves #48 by removing the need to (re)define KBUILD_OUTPUT in the package definition. Like I mention here, this was previously needed because kernel and kernelModuleMakeFlags didn't match, so KBUILD_OUTPUT had a value that didn't match the system kernel. Now they should match, thus removing the need to (re)define KBUILD_OUTPUT.

While the Chaotic Nyx project mentioned in the original issue seems to be deprecated now, I was able to test this with https://github.com/xddxdd/nix-cachyos-kernel which had the same behavior. Before the changes I got the same build error when I removed the KBUILD_OUTPUT, with these changes it builds normally.

Technically it would have been enough to add kernelModuleMakeFlags = config.boot.kernelPackages.kernelModuleMakeFlags; to the overrides, but I thought to also make the code a bit more (imo) idiomatic here.

This pr has multiple changes, but I split them to commits so hopefully that helps reviewing. The reason I removed the usage of the overlay from the NixOS module is that it felt a bit awkward first pass the "wrong" kernel and then override it manually later, instead of using config.boot.linuxPackages.callPackage that automatically provides the system's kernel and kernelModuleMakeFlags.

Ping @kitten

[delet-this]

Rebased

[keenanweaver]

Successfully built and tested on cachyos-latest-lto-zen4. Thank you very much!!

uname -a && sudo dmesg | rg yeet                    
Linux nixos-desktop 7.0.0-cachyos-lto #1-NixOS SMP PREEMPT_DYNAMIC Tue Jan  1 00:00:00 UTC 1980 x86_64 GNU/Linux
[   22.051176] yeetmouse: found a possible mouse (HID) Logitech G502 X LS
[   22.051183] yeetmouse: connecting to device: input13 (Logitech G502 X LS at usb-0000:14:00.0-2.2/input2:1)