Error compiling Nix packages from source

Issue: I’ve been trying to recompile Nix packages locally from source with various CPU optimizations, but have been running into a variety of errors when doing so. Goal: All I care about is getting packages compiled from source with my chosen compiler flags. I don’t care about how I do this (or even what compiler I use, though I understand Nix uses GCC). So if I’m going about this all wrong please let me know. What I have tried: In short, I’ve tried (and failed) to do this via (details later): ...