They don't want you forking it for that purpose, either.

https://github.com/NixOS/nixpkgs/pull/126326

Is it technically open source? Yes. Is it spiritually open source? No. Is it arrogance and poor management? Along with many ways this project is run, absolutely yes.

Use it but not modify it? With that attitude it can't even be classified as Open Source.

Hard pass.

Too bad. If they intentionally make it hard for people to make a software follow their distro conventions, be it immutability or reproceability, then they shouldn't be too surprised that forcing the software to behave properly could cause side-effects.